In guinea pigs the behaviour of male offspring is infantilized in adulthood, when their mothers had lived in an unstable social environment during pregnancy and lactation (Kaiser and Sachser, Psychoneuroendocrinology, 26 (2001) 503). The present study was conducted to elucidate the mechanisms bringing about this phenomenon. Therefore, the spontaneous behaviour and endocrine parameters of females were compared that either lived in a stable social environment (SSE) during pregnancy (SE-females) or in an unstable social environment (USE) during this period of life (UE-females). The SSE was made by keeping the group composition (one male, five females) constant; in the USE situation (one male, five females) every third day two females from different groups were exchanged. The spontaneous behaviour of the UE-females was recorded on three successive days: the day before, the day of and the day after the transfer from one group to another. The behaviour of the SE-females was recorded at corresponding times. In addition, serum concentrations of cortisol, dehydroepiandrosterone (DHEA) and dehydroepiandrosterone-sulfate (DHEAS) concentrations were determined. After transfer to another group UE-females showed distinctly more orientation behaviour and they received significantly higher amounts of courtship and sexual behaviour from the males than SE-females which remained in their familiar groups. In turn, UE-females displayed significantly higher amounts of urine spray towards the male, a defensive aggressive behavioural pattern. No differences were found in offensive aggressive and socio-positive behaviours between UE- and SE-females. With respect to endocrine parameters the transfer to an unfamiliar group did not cause an increase of cortisol, that is UE- and SE-females did not differ significantly. DHEAS and in part DHEA concentrations, however, were significantly lower in UE- than SE-females. Thus, for the first time it is shown that the social environment during pregnancy has significant effects on the androgen concentrations in female guinea pigs. Moreover, a decrease of androgens in pregnant females may be related to the infantilization of their male offspring.