Tested 2 gorillas, 2 orangutans, and 2 chimpanzees, all experienced Ss, on a multiple-problem 2-choice simultaneous discrimination task. A noncorrection procedure was used. 3 trials were given on all problems, and the number of intervening trials (filled with other problems) between Trials 1 and 2 (0, 2, or 10) and Trials 2 and 3 (2 or 10) of a particular problem was varied factorially. In addition there was a 15-20 sec. intertrial interval. The proportion of correct responses on Trial 2 decreased as the number of intervening trials between Trials 1 and 2 increased. Furthermore, as the number of intervening trials between Trials 1 and 2 increased, the proportion of correct responses on Trial 3 increased when Trials 2 and 3 were spaced (10 intervening trials), and decreased when they were massed (2 intervening trials). Results are discussed in terms of a unitary trace decay view and a stimulus fluctuation view, and both were seen to encounter difficulties. (PsycINFO Database Record (c) 2006 APA, all rights reserved)