The effects of sodium valproate (25-200 mg/kg i.p.) were investigated in the elevated plus-maze test of anxiety in the rat. No single dose significantly increased the percentage of entries made onto the open arms. Chlordiazepoxide (5 mg/kg) both acutely and after 5 days of administration significantly increased the percentage of entries onto open arms and this anxiolytic effect was neither potentiated, nor antagonised, by sodium valproate (25-200 mg/kg). Chlordiazepoxide also increased the total number of arm entries (indicating a stimulant effect). When valproate (25-100 mg/kg) was given together with acute chlordiazepoxide it produced a dose-related antagonism of this stimulant effect. The group receiving valproate (200 mg/kg) plus acute chlordiazepoxide (5 mg/kg) had a total number of entries significantly lower than the controls. Thus, the combination of 2 non-sedative doses resulted in a significant sedative effect.