Differences in the distribution of arginine vasopressin (AVP) and a subtype of AVP receptors, the V1a receptor, may explain dissimilarities in social behavior of monogamous and non-monogamous rodents. Intracerebroventricular infusions of AVP and a V1a antagonist were used in sexually naive males of two mouse species, the monogamous and highly aggressive California mouse (Peromyscus californicus) and the non-monogamous and less aggressive white-footed mouse (P. leucopus), to begin testing the interaction between the social system of a species and the effects of AVP on aggression. Two testing conditions, the resident-intruder aggression test (R-I) and the neutral arena aggression test, were used because they may differ in function and underlying biological mechanisms. In the R-I test, administration of the antagonist lengthened attack latencies in California mice. In contrast, blocking V1a receptors did not alter attack latencies in the R-I test in white-footed mice or in the neutral arena aggression test in both species. AVP also did not alter aggression in either species in either behavioral test. Overall, these results suggest that AVP may be more likely to be associated with offensive aggression as measured in the R-I test than with neutral arena aggression and that the effects of AVP manipulations may differ between monogamous and non-monogamous rodents. Aggress. Behav. 31:000–000, 2005. © 2005 Wiley-Liss, Inc.