Prairie deermice (Peromyscus maniculatus bairdii) possess a mid-ventral sebaceous gland pad which is larger in males than in females (Richmond & Roslund, 1952). In Syrian hamsters (Hamilton & Montagna, 1950), guinea-pigs (Martan, 1962) and Mongolian gerbils (Glenn & Gray, 1965), sebaceous gland complexes of males are known to require testicular or exogenous androgens in order to be fully active. The present report provides evidence that the ventral gland of the male prairie deermouse is under similar androgen control. Four individually housed male mice, 72–89 days old, were castrated and four were sham operated. On the day of the operations (week 0) and 1, 2 and 7 weeks later, the width of each animal's ventral gland was measured to the nearest 1 mm. (Gland length was not measured since the boundaries of the organ along the longitudinal axis are poorly defined.) The activity of each gland (as reflected by the amount