Effects of Micardis (Telmisartan), alone or with low-dose aspirin, on blood pressure and other cardiovascular endpoints are examined in 20 patients with MESOR-hypertension in a crossover, double-blind, randomized study consisting of three stages, each lasting 7 days: I-placebo, II-Micardis, and III-Micardis with low-dose aspirin. Treatment was administered each day at a different circadian stage, upon awakening, and 3, 6, 9, 12, 15 and 18 hr after awakening. During each stage, the following variables were measured at 3-hr intervals during waking: systolic and diastolic blood pressure, heart rate, ejection fraction, intrarenal resistive index, acceleration time, and serum creatinine. Each data series was analyzed by single cosinor. Results were summarized by population-mean least squares spectra. At matched treatment times, the MESOR and circadian amplitude of each variable were compared among the three treatments by paired t-tests. A prominent circadian rhythm characterizes all variables. Micardis was associated not only with a lowering of blood pressure, but also with a reduction of the circadian blood pressure amplitude. The ejection fraction was increased, and the resistive index and acceleration time were decreased, the effect being more pronounced when low-dose aspirin was added to Micardis. Any circadian-stage dependent effect of Micardis, with or without low-dose aspirin, will require monitoring over spans longer than a single day for a given treatment administration time.