A repeated acquisition procedure for the 8-arm radial maze was evaluated with trimethyltin (TMT). Ten male Long-Evans rats were trained to collect a single food pellet at the end of each baited arm on each trial of a daily 12-trial test session. Four of the eight arms were baited on all trials of a given session. The set of four baited arms was changed each day, thus requiring the rat to learn a new set of baited arms in each session. In trained rats, errors (entries into unbaited arms) declined from about 4 on Trial 1 to <1 on Trials 4-12 in each session: this within-session error reduction thus defined an acquisition baseline against which the effects of TMT were assessed. Learning was impaired after 7 mg/kg (iv) TMT, as shown by a slower decline in within-session error frequencies in all treated rats. Errors and response latencies were elevated for 5 weeks after TMT but returned to control levels thereafter. Histological examination of hippocampi showed damage to Ammon's Horn in all treated rats 18 weeks after treatment; however, no significant relationship between degree of damage and behavioral effect was observed.