The aim of the present study is to evaluate whether paroxetine (a selective serotonin re-uptake inhibitor) can modify the contractile responses of isolated vas deferens. Some contractile agents, potassium chloride (KCl), adenosine 5'-triphosphate (ATP), noradrenaline (NA), and electrical field stimulation (EFS) caused contractions both in epididymal and prostatic portions of vas deferens. Paroxetine (PX) in concentrations 10(-7) and 10(-6)M potentiated the contractions to KCl and ATP only in epididymal portion but in higher concentrations (10(-5) and 10(-4)M) inhibited the responses in both portions. NA responses were inhibited by PX in all concentrations used, both in prostatic and epididymal portions. Prazosin (PR), an alpha adrenergic receptor blocking agent, inhibited PX-induced potentiation observed for higher concentrations of KCl. PR also inhibited PX-induced potentiation on the responses to ATP in epididymal portion. Pretreatment with PX (10(-7) to 10(-6)M) increased the contractions to EFS but in 10(-5) and 10(-4)M concentrations inhibited them. Even though the preparations were washed out, the inhibited responses of contractile agents could not be restored. After a washout period for PX, when Bay K 8644 (calcium channel activator) was added to the bath medium, the contractile responses to KCl were partially restored. In calcium-free medium, KCl caused contractions in concentrations higher than 80 mM with lower amplitudes which were not affected by PX. Reserpinization did not change the inhibitory pattern of PX's effect on exogenously applied NA in all concentrations tested. In reserpinized rats, the potentiation caused by PX in exogenously applied ATP responses was not observed. In conclusion, we can say that PX has two different effects: inhibition and potentiation of contractions to various agonists. The inhibitory effect of the drug can be explained by a calcium channel blocking activity. The potentiating effect of the drug is mainly related to its presynaptic action, such as NA re-uptake inhibitory effect.