The record was studied of a 71-year-old, diurnally active (0700-2200 hours) male psychiatrist (G.N.) who self-measured systolic and diastolic blood pressure (BPS and BPD) mostly but not exclusively on Sunday mornings, from 1969 to 1994. A large about-yearly change was revealed which increased with age and was accompanied by a decreasing trend in the yearly rhythm-adjusted mean (MESOR; P < 0.01). According to conventional criteria that specify only upper limits of acceptability, G.N. was hypertensive in summer and normotensive in other seasons. Since changes in both MESOR and circannual amplitude occurred, a systematic surveillance of BP is the chronobiological recommendation.