Tolerance development to methadone in prenatally methadone-exposed male rats. (A) The latency and AUC of rats after receiving first injection of methadone. (B) The latency and AUC of rats after receiving 7th injection of methadone. (C) Rate of tolerance development to methadone in methadone or saline prenatally exposed rats. There was no difference in tolerance development to methadone (F(1, 31) = 0.535, p = 0.471) between the methadone and saline prenatally exposed groups. All data are expressed as mean ± S.E.M, (N = 4 per group).