Neuropeptide Y (NPY) has recently been shown be co-released with noradrenaline (NA) from sympathetic nerves and to cause arterial vasoconstriction in experimental animals and man. The effect of a single oral dose (10 mg capsule) of nifedipine on NPY- and NA-induced reductions of forearm blood flow (FBF) was studied in seven healthy volunteers. Intra-arterial infusions of NPY and NA into the brachial artery before nifedipine caused dose-dependent reductions in FBF with threshold doses of 0.2 and 0.03 nmol x min-1, respectively. The response to NPY was slower in onset and more long lasting than that to NA. Forty-five min after administration of nifedipine, FBF and heart rate had increased significantly (by 49% and seven beats x min-1, respectively, P less than 0.001), while no significant change was observed in systemic blood pressure. The NPY-induced decrease in FBF was slightly but significantly attenuated after compared to before nifedipine (19 +/- 6 vs. 28 +/- 5% at 1.0 nmol NPY x min-1; P less than 0.01). The response to NA was, however, not significantly altered by nifedipine. In conclusion, the NPY-induced reduction in FBF in man was only slightly prevented and the NA response not significantly affected by oral nifedipine administration in a clinically used dose. This suggests that this calcium antagonist, in the present dosage, does not, to any major extent, inhibit the vasoconstrictor effect of NPY or NA in man in vivo.