Using CO\sb2 anaesthesia together with a procedure minimizing pre-anaesthesia stress, a concentration of 10-30 ng/mL was found to be the serum corticosterone level of the quiescent adolescent and adult mouse near the nadir of the circadian rhythm. Dehydroepiandrosterone (DHEA) also was assayed in the serum of the normal healthy adolescent mouse, and was found to range between 0.3 and 2.5 ng/mL. The blood concentration of the sulfated form of DHEA was below the detection limit of the assay (1.9 ng/mL). The results define the physiological range of blood corticosterone and DHEA levels in the quiescent adolescent and young adult mouse. In a second experiment, weanling male and female mice were fed a complete diet ad libitum or were subjected to wasting protein-energy malnutrition (PEM) either through restricted intake of the complete diet or by ad libitum consumption of a low-protein diet for 3, 6, 9, 12 or 14 days (C57BL/6J strain), and for 6 or 14 days (CBA/J strain). The two malnutrition protocols were selected because they are known to produce depressed thymus-dependent immunity. The impact of PEM on the magnitude and speed of elevation in blood corticosterone level is greater than previously suspected. The levels increased 30-fold by day 3 of malnutrition and as much as 60-fold by day 6 and subsequent time points. Thus, the increase in corticosterone is of sufficient magnitude and speed to play a role in the initiation of PEM-associated immunodepression. PEM exerted no influence on the serum DHEA level in the mouse when imposed from weaning into adolescence.