Four-parameter, non-linear regression models were used to calculate the ED50 for morphine-induced suppression of SI during (a) early adolescence (PD 25), (b) late adolescence (PD 45) and (c) adulthood (PD 90) for BALB and B6 mice that were reunited with a same-sex conspecific. SI values and morphine doses are reported in the same manner as Figure 1. SI values for the saline-injected group at PD 25 were 67 ± 6.4 s and 106 ± 8.8 s for BALB and B6 mice, respectively. SI values for the saline-injected group at PD 45 were 89 ± 8.5 s and 109 ± 8.9 s for BALB and B6 mice, respectively. SI values for the saline-injected group at PD 90 were 71 ± 7.2 s and 86 ± 11.1 s for BALB and B6 mice, respectively. The ED50 values for BALB and B6 mice were significantly different at each of the two developmental time points during adolescence, but not during adulthood. Inter-rater reliability, rp = 0.92, d.f. = 671. All SI data are reported as the mean ± SEM.