Buspirone, MJ-13805 and MJ-13653 did not produce a '5-hydroxytryptamine (5-HT) syndrome' in rats at doses up to 20 mg kg-1. These drugs were very weak 5-HT uptake blockers (IC50 much greater than 10 microM) compared to drugs such as chlorimipramine. These drugs did not inhibit either monoamine oxidase (MAO)-A or MAO-B. The Ki values for these agents as inhibitors of [3H]-5-HT and [3H]-ketanserin binding to rat frontal cortex or hippocampal membranes were in the microM range, well above the brain concentrations achieved after an oral dose of 25 mg kg-1. Parenterally administered buspirone blocked apomorphine-induced stereotypy, inhibited the 5-HT syndrome elicited by 5-methoxy-N,N-dimethyltryptamine, and delayed the onset of p-chloroamphetamine induced behaviours.