The radioimmunoassayable concentration of insulin in isolated whey fractions of mammary secretion of sheep (n = 28) was measured. The concentrations were high on the last 2 days of pregnancy (550 +/- 80 and 580 +/- 77 microU/ml, respectively), higher on the day of parturition (780 +/- 71 microU/ml) and lower on the next day (290 +/- 54 microU/ml). During the following 4 consecutive days the concentrations gradually decreased to an almost basal blood level (14 microU/ml). The high insulin concentration in the mammary secretion during the periparturient period might be the result of intensified transfer of insulin through the blood-mammary barrier and increasing needs for this hormone by developing mammocytes and probably for the newborn lambs (n = 42) adapting to extra-uterine life. Conversely to the consecutive decrease in insulin, progressively increasing concentrations of sugars in mammary secretions were observed (from 2.1 +/- 0.22% on the second day prior to parturition to 5.5 +/- 0.13% on the fifth day post partum.