Adult male Long-Evans rats were intubated with either 0,5,6, or 7 mg/kg trimethyltin chloride. Activity was measured for 1 hr in a figure-eight maze 2 hr after dosing (day 0) and again on days 4,8,16 and 32 after dosing. On days 49-51, activity was measured in a figure-eight maze over a 23-hr period. There were no differences in activity on the day of dosing, but on all subsequent test days the 7 mg/kg TMT animals were hyperactive. TMT also altered the spatial pattern of activity: activity was increased in the 'figure-eight' portion of the maze but not in the blind alleys. Activity of the 7 mg/kg TMT animals was increased during all periods in the 23-hr test. Decreases in the length of the pyramidal cell line (CA1 to CA3c of the hippocampus) confirmed neuronal cell loss in TMT-dosed rats.