During observations in the home cage in the light and dark on days 5, 15, 25, 35, and 45, male guinea pigs mounted and exhibited anogenital investigation of the mother, though appropriately directed mounting occurred infrequently and was not shown by all males. Mothers nipped and kicked advancing males. No mounting of a female sibling was observed. When behavioral interactions of ∼35-day-old males with the mother or an unfamiliar adult female were compared during 1-hr tests in the home cage, males exhibited much higher levels of mounting and anogenital investigation, and received many more nips and kicks, when with the unfamiliar female. The results 1) demonstrate that limited maternally directed sexual behavior occurs in the home cage, 2) support earlier findings indicating that continuous housing with the mother suppresses maternally directed sexual behavior in periadolescent guinea pig males, and 3) suggest that sexual behavior toward female siblings also is suppressed. © 2003 Wiley Periodicals, Inc. Dev Psychobiol 42: 91–96, 2003.