Peripheral mononuclear leukocytes (MNLs) are widely used as a tissue model in studies of beta-adrenoceptor disturbances in hypertension and asthmatic diseases. The beta 2-adrenoceptor density (Bmax), however, depends not only on the gender of the person under study and on the time of day the blood specimens are obtained. Evidence is now reported for a circannual variation in the expression of beta 2-adrenoceptor sites on peripheral MNLs. In male volunteers the 24-h mean was found to be highest in the men studied in April/May (1135 +/- 10 sites/cell) and decreased to 891 +/- 16 sites/cell in August and to 712 +r90 sites/cell in December (means +/- SE, P less than 0.01 April/May compared to December). Concomitantly the circadian amplitude increased from 17.3% +/- 6.4% of 24-h mean in April/May to 28.2% +/- 1.4% of 24-h mean in August and to 34.2% +/- 4.2% of 24-h mean in December (means +/- SE, P less than 0.05, April/May compared to December). The circadian acrophase remained constant (190 degrees +/- 30 degrees equivalent to 12 h 40 min +/- 2 h 00 min, means +/- SE).