In order to understand the effects of solar irradiance on calcium metabolism we measured serum 25-hydroxycholecalciferol and total calcium levels monthly in 6 female nonpregnant reindeer maintained in the Oulu area (65 degrees N). Mean monthly serum total calcium levels varied slightly and the highest levels were seen in October. Serum 25-hydroxycholecalciferol levels were highest in October but the monthly variation was also slight. The small monthly variation of the analytes' abundance and the fact that they peaked not until 4 months after the longest day, (i.e. the main stimulus for vitamin D synthesis) could have something to do with the supplementation of this vitamin in the diet of the captive reindeer.