The aliphatic organo-metal derivative, trimethyltin, causes marked morphological damage to the central nervous system (CNS), when the compound is administered by the intragastric route. This report describes certain behavioral consequences of [CH3]3Sn treatment. Either 7.0 mg/kg [CH3]3SnCl or 0.9% saline was injected intragastrically in male rats of Long-Evans strain divided into two equal groups of nine each. Forty days later open-field activity during a 2-min interval was measured for each rat. Following activity testing, the animals were trained to press a lever for food reinforcement on an ascending fixed-ratio series ranging from FR 2 to FR 99. The results showed that rats treated with [CH3]3SnCl were three times as active as controls in the open field, and emitted lever responses at a significantly higher rate than controls throughout the fixed-ratio series regardless of the reinforcement schedule. The possible neuropathological consequences and the relative permanence of the neurobehavioral changes following tin treatment are discussed.