Results are described giving prolactin plasma concentrations, as determined by radioimmunoassay, in 5 non-lactating female and 4 male calves from a few days after birth to an age of 20 or 24 months. The blood samples were always collected at the same time of day (8–9 a.m.). Prolactin concentrations were shown to be dependent on the season, higher values being found in the summer, lower values in the winter. A significant correlation (r = 0.9; p