1. The role of the sympathetic nervous system and the effect of vasopressin (AVP) on the hypotensive action of nifedipine (Nf) were evaluated in conscious, unrestrained normotensive and DOCA–salt hypertensive rats.
2. The hypotensive response to Nf was much greater in DOCA rats than in the controls.
3. Solitary blockade of the sympathetic nervous system or AVP, did not alter the Nf effect in either DOCA or control rats. However, a combination clearly diminished the effect of Nf in the DOCA group, but enhanced it in the controls. The inhibition of angiotensin II (ANG II) augmented the hypotensive effect of Nf in control animals, but not in the DOCA rats. The percentage fall in blood pressure with Nf was much the same in both groups after the combined inhibition of the sympathetic nervous system and AVP.
4. The enhanced hypotensive action of Nf in DOCA rats may be dependent on the hyperactivity of the sympathetic nervous system and AVP, which facilitates calcium influx, and in the normotensive animals the depressor response to Nf may relate to blockade of the calcium influx, independent of the sympathetic nervous system, AVP and ANG II.