SummaryThe blood concentrations of aldosterone, corticosterone and cortisol were measured in conscious, non‐stressed guinea‐pigs using a double isotope dilution derivative assay procedure. Aldosterone levels in the guinea‐pig were high when compared with those of other species. The concentration of aldosterone, 37·9. ± 15·9 ng/100 ml (χ ± SD), and cortisol, 31·8 ± 10·1 μg/100 ml, found in non‐pregnant females on a moderate sodium intake was significantly greater than in males (aldosterone 22·2 ± 2·4 ng/100 ml and cortisol 19·3 ± 5·7 μg/100 ml). There was no sex difference in corticosterone concentration; females, 0·25 ± 0·06 μg/100 ml and males, 0·23 ± 0·10 μg/100 ml. The oestrus cycle had no effect on levels of the three steroids measured. Two thirds of the way through the 68‐day gestation period aldosterone levels were significantly elevated compared with non‐pregnant values (68·7 ± 50·9 ng/100 ml versus 37·9 ± 15·9 ng/100 ml, p<0·05). Values at day 20 (33·2 ± 11·7 ng/100 ml) and day 60 of gestation (51·9 ± 21·7 ng/100 ml) were similar to those of non‐pregnant animals. Cortisol and corticosterone levels were significantly elevated at 20 days gestation and they continued to rise until, at day 60, cortisol was 9 times and corticosterone 4 times higher than the non‐pregnant values. Compared with a moderate Na intake, salt loading suppressed aldosterone levels and Na restriction raised them.