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The normal output of urinary 17-OH corticosteroids was measured in male guinea pigs and its relationship to urine volume was established before and after treatment with ACTH. A diurnal rhythmicity was found with lowest corticosteroid excretion at night. By analysis of variance it was found that differences in corticosteroid output within individual guinea pigs was less than the variations of individual animals throughout a 3-day period. A definite correlation (γ = .75) was established between urine volume and corticosteroid output in the resting animal. Following treatment with ACTH, however, increased corticosteroid output was no longer associated with increased urine volumes (γ = .34). It is concluded that a direct relationship between corticosteroid output and urine volume exists only during lower levels of adrenocortical activity and, therefore, the assay of exogenous ACTH by corticosteroid output in the guinea pig is not invalidated by changes in urinary volume.