Weddell seals have been found to have an absence of diurnal patterns of secretion of the hormones cortisol and melatonin, the hormones which animals use to monitor the annual pattern of changes in daylight and thus regulate the physiological changes in their bodies. Blood samples also have shown that lactating females have unaltered metabolisms in spite of their state of fasting. The hormonal and ... metabolic changes occurring during the lactation period in these animals was investigated from a colony near Big Razorback Island, McMurdo Sound. Blood was taken from five lactating seals plus their pups and three non-lactating seals at 20 minute intervals for up to 12 hours to provide plasma samples for measurement of secretory profiles of growth hormone. Fifteen non-lactating females were blood sampled every 30 minutes for 4.5 hours to study the response to desipramine; a dynamic test of pineal gland function. Fifteen other lactating females were sampled once only. A dose response study of melatonin secretion following injection of desipramine to 6 non-lactating cows during the day and 6 others at night was also done. Blood was tested for chemical analysis. The information is relevant for understanding the control of rhythmic processes in ourselves and production animals.