We present a repeated-measures analysis of ethanol-induced loss of righting reflex (LRR) in Inbred Long-Sleep (ILS) and Inbred Short-Sleep (ISS) strains of mice and their F1 and F2 cross progeny. Mice were administered a 4.1 g/kg intraperitoneal dose of ethanol at two times, 7–10 days apart. Repeatability is nonsignificant in ILS, ISS, and F, mice, but is highly significant (0.47, p < 0.01) in the F2 mice. Mean LRR does not differ between trials 1 and 2, with the exception of the ISS strain in which the interaction of sex with LRR sensitization is significant. This two-trial method leads to increased accuracy of genotype assessment for pharmacological or behavioral traits where trial 1 does not influence the outcome of trial 2. The repeated-measures design facilitates novel analyses of the duration of LRR, and results suggest that most environmental variance for LRR is due to nonreplicable influences.