In 96 normal male sheep, we studied the variations with age of serum osteocalcin (bone Gla-protein), measured with an assay specific for ovine osteocalcin. We compared serum osteocalcin with the main histomorphometric parameters of bone growth measured on the metacarpus of 20 normal lambs from birth to 90 days of age. Serum osteocalcin significantly decreased with age (r = -0.70, p less than 0.001), particularly during the first 90 days of life (r = -0.85, p less than 0.001). During this growth period, serum osteocalcin was significantly correlated with the appositional rate (r = +0.73, p less than 0.001), the rate of longitudinal bone growth (r = +0.68, p less than 0.002), the rate of production of chondrocytes in the growth plate (r = +0.60, p less than 0.007), and the thickness of the growth plate (r = +0.79, p less than 0.001). In low birth weight male lambs (growth-retarded animals), serum osteocalcin was significantly lower at birth when compared to normal lambs (271 +/- 156 vs. 535 +/- 169 micrograms/l, p less than 0.001), and was also significantly correlated with histomorphometric parameters. We conclude that serum osteocalcin, which is already known as a sensitive and specific marker of bone formation, is also a sensitive biochemical marker of skeletal growth in normal and growth-retarded lambs. In addition, sheep appears as a valid animal for experimental studies on bone growth.