The experiment was carried out on 2.5-month-old calves. Prior to and during the experiment, the animals remained under the same light/dark regime (LD 10:14). Blood was sampled from the external jugular vein eight times during a 24-hour period, at 6.00 p.m., 9.00 p.m., midnight, 3.00 a.m., 6.00 a.m., 9.00 a.m., noon, and 3.00 p.m. Somatotropin and melatonin were assayed using radioimmunoassay. Blood concentration of melatonin revealed statistically significant differences between the dark and light phase. It was significantly higher during the dark phase. Changes in blood somatotropin concentrations did not depend on the phase of illumination. No correlation has been found between the hormones in terms of their concentration changes in calf blood over 24 hours.

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