Ren & Stimpy "Adult Party Cartoon" is an American adult animated comedy television series created by John Kricfalusi and produced by Spümcø for TNN / Spike TV. The series was developed as a more extreme revamp and spin-off of Nickelodeon's The Ren & Stimpy Show, whose first two seasons were produced by Spümcø.

The series premiered on June 26, 2003, and was removed from the network on July 24, after airing three episodes; the remaining episodes were released on DVD. During its short run, Adult Party Cartoon was heavily panned by critics, audiences and fans of the original series.

The original Ren & Stimpy Show premiered alongside Rugrats and Doug as one of the original Nicktoons on children's network Nickelodeon in 1991. The show's creator, John Kricfalusi, had many altercations with the network, eventually culminating in his termination the following year. The series continued until 1995 at Games Animation, with issues prevalent during Kricfalusi's tenure still present; it eventually ended with a single episode airing on MTV in 1996. In 2002, about a decade after Kricfalusi's termination, Viacom (which owns Nickelodeon) contacted him to produce a new version of his series for an updated version of sister network TNN/Spike TV, which was devoted to programming for male audiences. Kricfalusi said that TNN wanted an "extreme" version of The Ren & Stimpy Show. TNN gave Kricfalusi greater control of the writing and contents of the episodes, and he produced six "new" episodes aimed at adult audiences; of the episodes, "Altruists" was not a leftover from the original series, being an original idea from Eric Bauza. As in the original series, Kricfalusi ran into problems with meeting production deadlines and budgetary limits, with three out of the nine episodes ordered by the network being completed on time at the cost of the entire nine-episode budget initially allocated by Spike.

Several alumni from the original series returned to work on the new episodes, most notably co-developer Jim Smith, director and writer Vincent Waller as well as production assistant and writer Richard Pursel. Animation studio Carbunkle Cartoons returned to provide animation services, with Bob Jaques serving as animation director. Most Ren & Stimpy alumni who had joined Games Animation in the wake of Kricfalusi's 1992 firing, including series developer Bob Camp, had become estranged from Kricfalusi and thus were not involved with the revival; Vincent Waller, having joined the studio after the original series ended, was an exception. Some of the original voice cast members returned, with the exception of Billy West, original voice of Stimpy and second voice of Ren and Mr. Horse, who turned down offers to reprise his role as Stimpy as he did not consider the series to be funny and felt that participating in it would damage his career. Eric Bauza was hired to replace West as Stimpy, while Kricfalusi reprised his roles as Ren and Mr. Horse. Cast members Harris Peet and Cheryl Chase also returned, and Kricfalusi's father Mike Kricfalusi and long-time childhood friend Tom Hay provided some voices.

Pursel, who worked on the series as a storyboard artist and writer, despised the work experience, considering it to be his worst; Kricfalusi refused to communicate with him directly and would add shock value to his stories.

The new series began airing in June 2003 as part of an animation block also featuring Gary the Rat, Stripperella, and digitally remastered episodes of the original Ren & Stimpy series, subtitled "Digitally Remastered Classics". Kricfalusi directed the first episode, "Onward and Upward", based on requests from fans from the Nickelodeon era; Vincent Waller wrote the episode for the original series' second season but was rejected by Nickelodeon story editor Will McRobb. "Fire Dogs 2", a sequel to the original series episode "Fire Dogs" featuring Ralph Bakshi, was similarly rejected by McRobb before being remade in Adult Party Cartoon. The episode portrayed the characters as bisexual. Advertisers objected to some of the new show's content, particularly that of the risqué episode "Naked Beach Frenzy", which did not air in the show's original run, causing trouble with scheduling. The show stopped airing after three episodes when TNN's animation block was "put on hold".

The remaining episodes were set to resume in August 2004 along with the premiere of Spike's new animated series Immigrants (developed by fellow Nickelodeon partner Klasky Csupo), but both shows were pulled and never aired.

Kricfalusi shut down Spümcø shortly on July 18, 2005, thereafter following a lawsuit filed by Carbunkle Cartoons for failing to pay the animation studio for their services. In 2005, he announced that all of the Adult Party Cartoon episodes that were fully produced were coming to DVD, which was released by Paramount Home Entertainment as Ren & Stimpy: The Lost Episodes on July 18, 2006.