The zona glomerulosa of the adrenal gland is well developed in mice of the strain CBA/Fa Cam, but has a relatively poor development in mice of the Peru strain. Nevertheless, there was no significant difference between the strains in rates of aldosterone secretion, estimated both from in vitro incubations and from calculations based on metabolic clearance rates and measured plasma aldosterone concentrations. Plasma concentrations were lower in female mice than in male mice of the same strain, probably because of shorter plasma half-life in female mice. Plasma concentrations were less in CBA mice than in Peru mice of the same sex, probably because of the larger body weights and apparent distribution vol of CBA mice. These findings suggest that the ‘zonal theory’ of the adrenal cortex, according to which the zona glomerulosa simply represents the site of aldosterone production, may require some modification. (Endocrinology90: 968, 1972)