The actions of 5-hydroxytryptamine (5-HT) on the electrically induced twitch responses of mouse vas deferens were studied. 5-HT at the concentration range of 10(-8) to 10(-4) M produced a "bell-shaped" concentration-response curve on the field-stimulated twitch contractions; the enhancement of the contractions was maximum at 10(-5) M and progressively reduced at the concentrations of more than 10(-5) M. In the presence of ketanserin, whereas the stimulatory response to low concentrations of 5-HT (less than or equal to 10(-6) M) was not changed, that to high concentrations was reversed. The stimulation by 5-HT (less than or equal to 10(-5) M) was principally antagonized by MDL 72222. In the presence of both MDL 72222 and ketanserin, 5-HT inhibited the twitch contractions in a dose-dependent manner. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and BP-554 (1-[3-(3,4-methylenedioxyphenoxy)propyl]-4-phenyl piperazine), selective 5-HT1A agonists, only inhibited the twitch contractions. Downward slope of the contraction-response curve of 5-HT (greater than or equal to 10(-5) 5 M) was shifted to right in the presence of 8-OH-DPAT. 5-HT and 8-OH-DPAT had no effect on the tension of unstimulated organs. Contractions elicited by ATP were potentiated by 5-HT, which was antagonized by ketanserin. 8-OH-DAPT did not affect ATP-elicited contractions. These results suggest the presence of presynaptic 5-HT1, maybe 5-HT1A and 5-HT3 receptors mediating inhibition and potentiation, respectively, of neurotransmitter release and of postsynaptic responsible for enhancing neurogenic contractions in mouse vas deferens.