Suckling rats from 5 to 30 days of age were subjected to fasting in a 37° C chamber to avoid possible metabolic effects from low environmental temperature. The percentage of body weight loss in 24 h fasting increased along with the age of the rats. Blood glucose levels were the same in 5, 10, 20 and 30 day old animals when fed, whereas fasting produced a fall in all the groups which was minimal in the 20 day old animals. Plasma insulin levels were rather low in 10-day-old fed animals; the maximal decrease in this parameter was reached by 5-day-old rats under fasting. Both beta-hydroxybutyrate and acetoacetate levels were higher in animals of 5 and 1 0 days of age than in those of 30 days, but fasting did not produce any changes in the former while those parameters augmented in the 20 and 30-day-old animals. The results are discussed in relation to the high fat content in the mother's milk. which affects the metabolic situation of the suckling rats when fed and their response to the fasting situation.