Ambulatory monitoring of intra-arterial blood pressure was used to assess patterns of circadian blood pressure in 12 hypertensive patients who were treated with pindolol in either a once- or twice-daily dosage regimen. Neither once-daily nor twice-daily pindolol had much effect during the latter part of the night and early morning. In the six patients who agreed to further crossover studies, the hypotensive effects of once-daily therapy were not significantly different from those produced by a twice-daily regimen. While confirming the effectiveness of once-daily beta blockade in hypertension, we deduce that failure to affect nighttime blood pressure substantially is a feature independent of the dosage regimen and also, probably, of the individual beta blocker used.