The effect of baclofen on the locomotor activity of control and small-platform-stressed mice was studied. In the small platform technique, mice are forced to stay on small platforms (d= 3.5 cm) surrounded by water for 24 h. Small platform stress increased the locomotor activity of mice in the actometer. Baclofen administered at doses of 0.25, 0.5 and 1.0 mg kg(-1)(i.p.) had no effect on the locomotor activity of control mice. In small-platform-stressed mice, the locomotor depressant effect of baclofen was pronounced, being statistically significant at a dose of 1.0 mg kg(-1). These data suggest that small platform stress induces hypersensitivity of mice to the motor depressant effect of baclofen. On the basis of these data it could be proposed that small platform stress induces changes in the function of GABA(B)receptors and that GABA(B)receptors participate in the behavioural changes caused by small platform stress.