The authors examined the effect of different classes of females on the plasma cortisol response of male guinea pigs (Cavia porcellus) from shortly after weaning through full adulthood. Males housed under standard laboratory conditions with artificial harems of 2 unrelated females of similar age were tested at 4 age ranges (~Days 40, 120, 180, and 240). At each age range, males were placed into a novel environment for 2 hr on 4 separate occasions: while alone, with 1 female cage mate, with the other female cage mate, or with an unfamiliar female. Relative preference for the 2 female cage mates was determined from observations in the home cage. Puberty was estimated by plasma testosterone levels. At the 3 youngest ages, the more preferred, less preferred, and unfamiliar females did not differentially reduce the plasma cortisol response. At 240 days of age, only the presence of the more preferred female was able to significantly reduce cortisol levels. These results demonstrate a very selective social buffering effect on the plasma cortisol response in a nonmonogamous species. This effect emerges well after sexual maturity and can be observed under standard laboratory housing.