Norepinephrine (NE) overflow from field-stimulated rat was deferens preparations was quantified directly by electrochemical detection using high performance liquid chromatography. The effect of agonist (BHT 920) and antagonist (rauwolscine) of prejunctional alpha 2-adrenoceptors on NE overflow was assessed and compared with their effect on the smooth muscle mechanical response to field stimulation. Increasing the stimulation frequency from 2 to 30 Hz resulted in an increase in muscle tension together with an increase in NE overflow. Addition of 1 microM rauwolscine to the medium resulted in a significant increase in muscle contraction to field stimulation which reached a maximum at 5 Hz. On the other hand, NE overflow increased linearly with the frequency of stimulation within the range studied. Addition of 0.1 microM BHT 920 to the medium significantly decreased the amplitude of contractions at lower stimulation frequencies (2 to 10 Hz) but elicited no significant changes at high frequencies. BHT 920 did not significantly affect NE overflow for all range of stimulation frequency. The simultaneous recording of field-stimulation induced contractions and NE overflow indicates that in the rat vas deferens, rauwolscine acts like a pure alpha 2 adrenoceptor antagonist at a prejunctional level. BHT 920 did not appear to affect selectively prejunctional alpha 2 adrenoceptors but also may activate postjunctional alpha 1 adrenoceptors.