Previous hormonal studies have identified the hormonal and stimulus factors mediating the initiation of maternal behavior but have failed to reduce hormone-induced latencies of nonpregnant females to less than 1-2 days of continuous pup exposure. For the purpose of testing whether this delay is due to an olfactory-vomeronasal-mediated aversive reaction to pups like that found in untreated virgins, estrogen-injected hysterectomized-ovariectomized (HO-EB) nonpregnant females were subjected to olfactory-vomeronasal deafferentation. Median latencies were reduced to 0 and 1.5 hr compared with 72 hr for HO-EB nondeafferented females, which indicates that normally after HO-EB treatment, tendencies to initially avoid pup contact remain strong. Next, the hypothesis was explored that experiences during late pregnancy and/or parturition interact with hormonal priming to modify pup avoidance. Nonpregnant HO-EB females that had been exposed to pregnant-parturient females for 2 wk were tested under conditions simulating parturition. A high percentage rapidly initiated maternal behavior, but conditions during testing proved more important than prior exposure to pregnancy/parturition. The contributions of three stimulus/situation factors were analyzed: pup age--newborn versus 3-8 day; method of pup presentation--four at once versus staggered introduction; and time of day--lights versus dark phase. Hormonally treated but not sham-treated females initiated maternal behavior most rapidly when first exposed to one newborn in the nest during the light phase (76% within 1.5 hr). Prepartum caesarean-delivered females, however, responded maternally to four 3-8-day-old pups outside the nest, which indicates that additional factors operate at parturition.