The vitamin D status was determined on one to four occasions either after summer (September-October) or after winter (March-April) in 175 male adolescents (13-17 years), resulting in 394 measurements of serum 25-hydroxyvitamin D (25(OH)D) and intact parathyroid hormone (iPTH). The subjects lived in a rural area to the north of Paris (49 degrees N). After summer the 25(OH)D concentration was 58.5 +/- 18.0 nmol/l (mean +/- SD), while after winter it had fallen to 20.6 +/-6.0 nmol/l (p = 0.0001). Meanwhile the iPTH concentration was 2.76 +/- 0.97 pmol/l (mean +/- SD) after summer and increased to 4.20 +/- 1.21 pmol/l after winter (p = 0. 0001). All the results were pooled and a nonlinear population model with random parameters was used to describe the relationship between serum iPTH and 25(OH)D. When the concentration of 25(OH)D was higher than 83 nmol/l, an iPTH mean 'plateau' level at 2.48 pmol/l was reached. When 25(OH)D concentrations fell below 83 nmol/l, the increase in iPTH concentration accelerates, and when the mean 25(OH)D concentration was equal to or lower than 10 nmol/l the mean iPTH level (4.97 pmol/l) was twice as high as the 'plateau' value.