Previous studies indicate that subacute toluene exposure enhances the effects of postsynaptic doses of apomorphine on locomotor activity in the rat. We have now studied the effects of the ganglioside GM1 on toluene-affected apomorphine-induced (1 mg/kg, s.c.) locomotion, motility, and rearing. Treatment with GM1 (10 mg/kg, i.p., 1 h before exposure) was found to counteract or even reverse the enhancing effect of toluene on apomorphine-induced locomotion and rearing, but had similarly to toluene no significant effects on apomorphine-induced motility or on spontaneous locomotor activity. The antagonistic effects of GM1 may be due to its ability to block toluene-induced changes in D2 receptor binding.