Mice from 15 standard inbred strains were tested for sensitivity to two effects of acute morphine administration, open-field activity, and body temperature changes, at doses of 0, 4, 8, 16, and 32 mg/kg, I.P. Large strain differences were consistently observed, indicating a substantial degree of genetic determination of these traits. For morphine-induced activity, some strains were markedly insensitive to all doses (e.g., C3H/He, CE), while others showed increases and some decreases at the same morphine dose. For thermal responses, one strain was insensitive to all doses employed (C3H/He), while others showed marked hypothermia and some hyperthermia at the same dose. Although strains differed in brain morphine concentrations at time of behavioral testing, pharmacokinetic differences were unrelated to both measures of morphine sensitivity. Correlations among strain means (estimates of genetic correlations) were rather high across doses within each measure, indicating that strain differences to a given effect of morphine were rather stable across doses. This suggests substantial commonality in genetically mediated mechanisms across the dose range used for activity, and also for thermal responses. In contrast, genetic correlations between activity and thermal responses were not significant at any dose, indicating that these two traits are largely genetically independent.