4-Aminopyridine (4-AP) acts by blocking voltage dependent potassium channels in excitable membrane thus increasing the influx of extracellular (Ca2+) in response to an action potential. 4-AP exerts either in vivo or in vitro effects in many organs. The present report deals with a study on the electrically stimulated rat vas deferens of the action of 4-AP on Ca2+ mediated mechanism related to the adrenergic transmission. To avoid the interference of the non-adrenergic components we used the epididymal portion of the vas. 4-AP was shown to enhance the electrically-stimulated contractions in a way which was inversely related to external Ca2+. This effect was abolished by nifedipine (1 x 10(-6) M). Unexpectedly, however, 4-AP acted synergistically with nifedipine by enhancing the inhibition of the electrically-induced tension sustained by nifedipine. This property of 4-AP was shared by noradrenaline, phenylephrine, clonidine, serotonin, morphine and ATP. Their inhibitory effects were antagonized by prazosin (1 x 10(-8) M), yohimbine (2.5 x 10(-5) M), methysergide (1 x 10(-4) M), naloxone (1.2 x 10(-7) M), and theophylline (6 x 10(-6) M) respectively. The inhibitory effect of 4-AP was antagonized only by prazosin (1 x 10(-8) M) and was not observed in vas deferens of rats pretreated with reserpine. It is concluded that 4-AP is able not only to activate excitatory mechanisms but also to stimulate inhibitory alpha 1-adrenergic mechanisms.