Effects of captopril, clonidine, hydralazine, metoprolol, and prazosin on the circadian variability of blood pressure (BP) and heart rate (HR) as well as on BP and HR rises at awakening were studied in spontaneously hypertensive rats (SHR). An on-line computerized system was used for continuous intraarterial (i.a.) recording of BP and HR. Drugs were infused intravenously (i.v.) with osmotic minipumps. Circadian BP patterns were more pronounced and variable in SHR than in Wistar-Kyoto normotensive control rats. HR patterns were similar in both groups. Antihypertensive treatment resulted in differential effects on the expression of the circadian BP and HR patterns in SHR. Reductions in BP variability and reductions in the awakening increase in BP were not correlated with the BP-lowering efficacy of agents. Treatment with clonidine reduced both circadian variability of BP and HR and the awakening increase. Prazosin reduced circadian BP variability and its awakening increase, whereas metoprolol reduced circadian HR variability only. Hydralazine and captopril did not cause any major effects on the circadian variability of BP and HR. These data indicate that circadian rhythms of BP and HR are under sympathetic control in SHR and are not influenced by nonsympatholytic vasodilators.