[en] In 10 normal males aged 23-50 years measurements of serum alkaline phosphatase (s-AP) and the 24-h whole body retention of 99mTc-diphosphonate (WBR), as indices of bone formation, and the fasting urinary hydroxyproline:creatinine ratio (OHPr:Cr), as an index of bone resorption, were performed monthly from January 1983 to May 1984. Bone mineral content of the distal forearm (BMC) was measured in the middle of each quarter. From January to May BMC exhibited a reproducible, significant average increase of 2.5%, returning to baseline level between May and August. During the first quarter of both 1983 and 1984 a significant increase in s-AP and WBR was seen. Subsequently, during the second quarter of 1983, these variables fell below the mean of the year. Confirming their interrelationship, the deviations of s-AP and WBR were positively correlated throughout the study period (r = 0.51, P less than 0.05). Since the urinary OHPr:Cr ratio remained constant, the reported seasonal changes in bone mass of normal, adult males appear to result from primary changes in bone formation