Twenty-four calves of the Holstein-Friesian breed were included into a trial. The trial was performed from April (when the calves were born) to October. The objective of the study was to investigate variations in insulin and glucose levels over a longer time period of the postnatal development of calves. The blood was sampled by the jugular vein puncture at the age of 1, 2, 3, 5, 14, 21, 28, 35, 49, 63, 91, 118, 147 and 175 days. The calves were housed in a prophylactorium calf-house till the age of 21 days: till the age of 14 days they were housed individually and then in groups of 2-3 calves in a small pen. The calves were fed milk till the age of 26 or 28 days, after a fortnight the feed intake was increased from the previous 3 litres to 5-6 litres per head/day. At the age of 21 days the calves were transferred to pens in the calving room. At the age of 26 days twelve calves were transferred to a barn for young cattle rearing, the remaining twelve calves were transferred at the age of 28 days; here they were housed by 10-12 head per pen and they were fed a commercial milk replacer diluted at a 1:10 ratio, the feed intake being 6-8 litres a day, and a commercial pelleted calf starter TKS at the amount of 1.0-1.1 kg.(ABSTRACT TRUNCATED AT 250 WORDS)