1 About 45% of singly-housed male mice showed timidity (alert postures, running away, defensive postures) instead of aggression on interactions in pairs with group-housed male mice, though their partners did not show any aggression. The isolation-induced timidity was stable in repeated interactions. Timid mice also showed locomotion (walking across cage and rearing) and a small amount of sociable activity (sniffing, following partners and climbing over them). 2 Diazepam (5 mg/kg), chlordiazepoxide (20 mg/kg), chlorpromazine (7.5 mg/kg) and barbitone (60 mg/kg) given orally inhibited the isolation-induced timidity without reducing other motor activities in the timid mice. Imipramine lessened timidith only in a dose (80 mg/kg) which also decreased other components of behaviour in the timid isolates.(+)-Amphetamine and lysergic acid diethylamide (LSD) increased the timid response. 3 Comparison of the inhibition of timid activities with changes in other behaviour occurring at the same time seems a better measure of selective timidity-reducing effects of drugs than the rota-rod test. 4 Diazepam (5 mg/kg) increased sociable and locomotor activities. Barbitone (20 and 60 mg/kg) increased sociable activities; however, the higher dose also evoked some aggression in timid mice. 5 Behaviour of timid singly housed male mice seems to be a good measure for prediction of activity of drugs in relieving anxiety as well as for detection of aggression-evoking and sociability-increasing effects of drugs.