The behavioral effects of trimethyltin were studied in male White Carneaux pigeons using the repeated acquisition procedure, which required that the birds acquire a different four-response position sequence from day to day. After rates and patterns of acquisition of new sequences had stabilized, trimethyltin was administered i.m. at three different dosages (1.0, 1.3 and 1.75 mg/kg) to three different groups of birds, and saline was administered to a fourth group. The birds received one injection and then were tested daily for a period of up to 10 weeks. Trimethyltin produced increases in total errors within sessions, as well as decreases in rates of responding. Analysis of the pattern of error elimination within sessions suggested that error elimination did take place after trimethyltin administration in all birds, but that errors frequently were eliminated more slowly than in control birds. Over the 10-week testing period, there was a gradual recovery to control levels of both total errors and the pattern of error elimination within sessions.