Serum osteocalcin (OC), which is a specific, sensitive, and rapidly responding biochemical bone marker of osteoblastic activity, varies in a circadian rhythm in both normal adults and children with the highest levels during nighttime. In the present study, we characterized the 24-h rhythm of serum OC in 8 growth hormone (GH)-deficient children without treatment, and during two regimens of recombinant human GH (r-hGH), i.e. 2 IU/day s.c. at 08:00 h or at 20:00 h, in comparison with 5 normal, untreated children. Without treatment and when r-hGH was administered at 08:00 h, the 24-h profile of serum OC was correlated with that of the healthy controls in only 1 of 8 patients (r = 0.26 +/- 0.09 (mean +/- SE) and r = 0.10 +/- 0.08) whereas the correlation was significant in 6 of 8 patients (r = 0.44 +/- 0.05) during evening r-hGH administration. Within GH-deficient patients, mean OC level was lower during no r-hGH or evening administration compared with morning administration. In conclusion, untreated GH-deficient children have an abnormal 24-h serum OC profile. Evening administration of 2 IU of r-hGH/day s.c. tends to normalise the 24-h profile but not the average daily level, whereas morning administration of GH increases average daily OC level but does not normalize the circadian rhythm.