A genetic analysis of differences in reactivity to early exposure of female BALB/cBy and C57BL/6By mice to testosterone was performed using the C x B recombinant-inbred (RI) strains. RI female mice were given a single injection of 1 mg of testosterone propionate on day 4 of life and observed in adulthood in the presence of a male attempting to mate. The latency of the first attack, the number of attacks per minute, the incidence of attack, and tail-rattling were recorded. The strain distribution patterns of the RI strains for all variables is consistent with polygenic correlate of reactivity to neonatal androgenization and is concordant with the heterotic inheritance effect observed in F1's for the latency of the first attack and the number of attacks per minute. The RI strain distribution pattern shows that the genetic correlates of reactivity to early testosterone exposure differ according to the variable studied.