The effects of amlodipine on blood pressure profiles, cardiac hypertrophy, and beta-adrenergic signal transduction were studied in transgenic hypertensive TGR(mREN2)27 rats (TGRs), which are characterized by an inverse circadian blood pressure rhythm. Cardiovascular parameters were monitored by radiotelemetry; beta-adrenoceptor density and function were measured by radioligand binding and by determination of beta-adrenergic stimulation of adenylyl cyclase. Ventricular weight and the activity of cardiac sarcolemmal 5-nucleotidase were used as measures of hypertrophy. Acute i.p. injection of amlodipine (1, 3, 10 mg/kg body weight) either at 8:00 or at 20:00 h dose-dependently reduced blood pressure irrespective of the dosing time. For long-term treatment, TGRs were divided into three groups: untreated; amlodipine, once-daily, 5 mg/kg; and amlodipine, twice daily, 2.5 mg/kg. Both treatment schedules resulted in decreased 24 h means in systolic and diastolic blood pressure and a reduction in ventricular hypertrophy but had no effects on cardiac beta-adrenergic signaling. Once-daily dose of amlodipine at 8:00 h decreased blood pressure predominantly during the daily resting period of the rats, whereas twice-daily dosing induced a bimodal blood pressure pattern. However, even after 5 weeks of treatment, typical circadian profiles could not be observed with either treatment, indicating a short duration of action of amlodipine in rats. Thus it remains an open question whether pharmacologic normalization of the circadian blood pressure pattern in TGRs will more effectively reduce myocardial hypertrophy and restore beta-adrenergic signaling than a reduction in 24-h blood pressure per se.