The behavioural development of male guinea pigs was studied in two colonies of 12 males and 12 females each, from their 30th to their 360th and 540th days of age respectively, living in enclosures of about 8 m2. Changes in plasma-testosterone (T) concentrations were determined in individually (N = 8) (IM) and colony housed males (N = 10) (CM) from their 30th to their 360th days of age. In IM T-titers did not change significantly over time. In CM T-titers increased significantly from 30 to 90 and 120 days of age respectively, declined to lower levels until day 210, rose significantly at 240 days of age and remained on relatively high levels until day 360. CM showed significantly higher T-titers than IM on days 90, 120 and 240. In CM the first T-peak might reflect hormonal puberty. At this time CM, however, displayed low amounts of courtship and agonistic behaviour. The 2nd T-peak (240 days of age) occurred at an age when CM could achieve high ranking social positions for the first time which allowed reproductive success. It might be caused by the intense increase in sexual and agonistic behaviour during this period. In individual CM no general relationship between behavioural and endocrine development was found. At all ages T-titers poorly reflected behavioural differences between CM. Overall, androgen development did not follow a fixed temporal pattern during ontogeny but was strongly influenced by the social environment.