Previous studies have found that combined lesions of the perirhinal and entorhinal (PRER) cortical areas do not impair, and in fact may facilitate, acquisition of successive olfactory discriminations. The present study sought to determine the effect of PRER lesions on the acquisition of simultaneous olfactory discriminations. Rats (N = 24; 9 PRER-lesioned, 15 sham-operated controls) were trained on a single simultaneous olfactory discrimination; PRER-lesioned animals were dramatically and persistently impaired in acquisition of the discrimination relative to sham-operated subjects. These data are consistent with the view that, in concert with the hippocampus, these cortical regions participate in the encoding of relations among multiple extant stimuli.