During extinction, groups of rats were confined in a goal box at the end of runs in a linear maze. Running times increased as the retention interval increased from 3 to 7 sec. and reached an upper limit determined by a group stopped just short of the goal box. Running time was a U-shaped function of retention time. A 2nd study showed that these results were not due to goal box aversiveness as a function of time or differential reinforcement as a function of the delay in return to the home cage. From Psyc Abstracts 36:01:1EL72K. (PsycINFO Database Record (c) 2006 APA, all rights reserved)