Contractile responses of the circular smooth muscle of the guinea-pig vas deferens to electrical stimulation were recorded isometrically using ring preparations of about 2 mm in breadth. Stimulation of low frequency (2.5 to 10 Hz, 0.05 msec duration) for 20 sec produced twitch contractions which occurred once or repetitively during the application of stimulation. Higher frequency stimulation (20 to 80 Hz) produced biphasic contractions, an initial phasic and a secondary tonic contraction. These contractions were abolished by 3 X 10(-6) M tetrodotoxin or 3 X 10(-5) M guanethidine; however, 3 X 10(-6) M atropine or hexamethonium did not affect the contractions. Prazosin at 10(-6) M, like 3 X 10(-7) M yohimbine, increased the amplitudes of twitch contractions to the low frequency stimulation and caused the twitch contractions to occur repetitively. On the other hand, prazosin suppressed the tonic contractions to the high frequency stimulation without substantially inhibiting the phasic contractions. Cocaine at 3 X 10(-6) M potentiated the twitch contractions in the presence of yohimbine. After in vivo reserpine treatments, the low and high frequency stimulation produced twitch and phasic contractions, respectively; however, tonic contractions were not induced. Prazosin at 10(-6) M did not qualitatively affect the contractions in preparations from the reserpine-treated animals. These results suggest that the neurogenic contractions of the circular muscle of the guinea-pig vas deferens are sympathetic in nature, but that they are not mediated solely by norepinephrine. Co-release of other transmitters was indicated to occur upon the electrical stimulation of wide frequency range.