Radioimmunoassayable growth hormone (GH) was determined fed and fasted female rats of different ages. In 6-40 day old rats blood was collected at hourly intervals in groups of rats at different time intervals during the day. Within each age group the variation in plasma GH was considerable. In 6-14 day old rats plasma GH was generally elevated. By day 18, levels declined, lowest on day 22 and by day 26 again increasing. In 14 day old rats the median plasma level of GH was 22 ng/ml, in 22 day old rats < 5 ng/ml and in 40 day old rats 43 ng/ml. In 14 day old rats levels ranged from < 5 ng/ml - 148 ng/ml, in 22 day old rats from < 5 ng/ml - 34 ng/ml and in 40 day old rats from < 5 ng/ml - > 200 ng/ml. A 20 h fasting period was associated with a significant decrease in plasma GH. In 45 day old rats, the variations in plasma GH of individual animals were studied by obtaining sequential blood samples from unrestrained, undisturbed animals with implanted intra-aortic cannulae. In these rats GH secretion was characterized by an episodic release, occurring every 2-4 h. After a 20 h fasting period major peaks were depressed and occurred less frequently. Apparently there is an agerelated as well as a circadian rhythm in growth hormone secretion in the rat and sequential sampling of blood is essential for the evaluation of the secretory pattern.