Vasoactive intestinal polypeptide (VIP) was infused into normal volunteers and patients with hyperprolactinaemia. Heart rate increased from 62 +/- 3 to 75 +/- 3 beats min-1 (P = 0.001) in controls and from 70 +/- 2 to 78 +/- 3 beats min-1 (P = 0.001) in hyperprolactinaemics. Similarly, haematocrit increased from 38 +/- 2 to 44 +/- 1% (P = 0.001) and from 40 +/- 1 to 43 +/- 2% (P = 0.002) and plasma renin activity from 910 +/- 59 to a peak of 3344 +/- 282 pg ml-1 h-1 (P = 0.001) and from 1577 +/- 671 to a peak of 4954 +/- 1364 pg ml-1 h-1 (P = 0.001) in the two groups, respectively. Prolactin concentrations rose in the control group only, from 134 +/- 11 to a peak of 377 +/- 35 mU 1(-1) (P = 0.001), whilst in the hyperprolactinaemics little change occurred from the pre-infusion concentration of 3873 +/- 2179 reaching a peak of 3998 +/- 2347 mU 1(-1) (P greater than 0.07). In separate studies, the normal subjects were pretreated with either bromocriptine or dexamethazone. Dexamethazone did not alter any parameter of the response to VIP. Bromocriptine did not affect the heart rate, haematocrit or renin response to VIP but clearly inhibited the rise in prolactin which remained at unmeasurable concentrations throughout the infusion.