The objective of this study was to examine the effects of dihydropyridine calcium antagonist therapy on 24-h baroreflex sensitivity. Twenty-three patients with moderate essential hypertension were studied before and during acute (10 patients) and chronic (21 patients) treatment with a dihydropyridine calcium antagonist (nifedipine, nicardipine or felodipine) as monotherapy in a dose titrated to produce a fall in mean cuff pressure of at least 10%. Twenty-four hour unrestricted ambulatory intra-arterial blood pressure (IABP) and heart rate (R-R interval) were monitored. Baroreflex sensitivity (BRS) was assessed throughout the 24-h period by off-line computer analysis of spontaneous variations in IABP and R-R interval. During acute first dose treatment with a calcium antagonist there was a significant fall in blood pressure (BP), increase in heart rate and reduction in BRS. With chronic therapy (6-16 weeks) there was a continued reduction in mean BP of 11% (P < 0.001), but heart rate had returned to control levels and BRS was significantly increased over the 24 h by 14% (P < 0.01). The increase in BRS was evident during both the waking and sleeping periods, but the greatest increase was during sleep (awake 12% P = 0. 02, asleep 28% P = 0.003). In conclusion, although dihydropyridine calcium antagonists acutely cause a reflex tachycardia associated with a reduced BRS, there is no such effect with chronic therapy. BRS was significantly increased after chronic treatment, with exaggeration of the diurnal pattern. Journal of Human Hypertension (2000) 14, 189-194.