['Differences in mu-opiate receptor (MOR) gene expression may modulate the rewarding effects of ethanol.', 'The effects of MOR gene knockout (KO) were examined in wild-type (+/+), heterozygote MOR KO (+/-), and homozygote MOR KO (-/-) mice on voluntary ethanol consumption, conditioned place preference produced by ethanol, and locomotor responses to ethanol in separate groups of mice.', 'Voluntary ethanol consumption (2-32% v/v) was examined in a two-bottle home-cage consumption test. The conditioned place preference paradigm was a biased design. Mice received four pairings of ethanol (2.0 g/kg IP) on the initially preferred side and four pairings on the initially non-preferred side with saline. The difference in time spent on the initially non-preferred side (pre- versus post-conditioning) was the measure of drug-induced preference. After habituation to a novel locomotor test chamber mice were tested, on subsequent sessions, for ethanol induced locomotion (0.0, 0.5, 1.0, and 2.0 g/kg IP).', 'Heterozygous and homozygous MOR KO mice consumed less ethanol than wild-type mice. These effects appeared to be greater in female KO mice than in male KO mice. MOR KO mice, especially females, exhibited less ethanol reward in a conditioned place preference paradigm. These effects on ethanol reward were produced by reductions in MOR expression levels as small as 50%. MOR KO mice exhibited less ethanol-stimulated locomotion than did wild-type mice, an effect that was also largest in females.'] Are ethanol consumption and reward decreased in mu-opiate receptor knockout mice?