Growth Hormone (GH) and serum Bone GLA-Protein (BGP or Osteocalcin), a sensitive and specific marker of bone turnover, were measured in 5 children with growth retardation, during 12 h period from 8 p.m. every 30'. A nocturnal periodicity in Osteocalcin was found: BGP rose slightly during sleep in the patients studied, maximum concentration being reached between 3 and 5 a.m. There were no consistent correlations between Osteocalcin concentration and circulating levels of GH in the subjects tested. BGP determination may be of interest in the evaluation of children with short stature but standardized analytical conditions remain to be determined.