Recent theories of hippocampal function focus on its role in the formation of associations in the temporal domain. A reversal learning paradigm based on leverpress automaintenance was developed to vary the CS-UCS relationship along two independent dimensions, one temporal and one not: CS+-UCS delay and the probability of reinforcement (P(RFT)) following the CS+. Eight male hooded Long-Evans rats were trained to reverse the automaintained discriminations repeatedly, until stable reversal performance was achieved. The neurotoxicant trimethyltin (TMT) was used to induce lesions in the CNS, including the CA3-4 region of Ammon's Horn in dorsal hippocampus. Following iv injection of 7 mg/kg TMT to half the rats, reversal performance was assessed under varying conditions of delay and P(RFT). After recovery from the acute effects of TMT (1-2 weeks), treated rats reversed normally when no delay separated the CS+ and UCS; with delays of 2 to 4 sec, they reversedless completely within a session than did controls. The degree of behavioral impairment correlated significantly with hippocampal damage only at nonzero CS+-UCS delays.