Analgesia and locomotor activity are genetically differentiated in C 57 and DBA mice. In fact, DBA strain, unlike C 57, is very sensitive to the analgesic effects of morphine. On the contrary, morphine elicits an increase of locomotor activity only in C 57 mice. We have used this genetic approach to study the in vitro response of vas deferens contractions to morphine and methionine-enkephalin. The results obtained are the following: 1. The percentage of morphine inhibition of the electrically evoked contractions of the longitudinal muscle of the vas deferens is greater in DBA strain, which is sensitive to the analgesic effects of morphine, than in C 57 mice in which morphine exerts a stimulating effect of the locomotor activity; 2. Met-enkephalin has been found to be more active than morphine on the same preparation; 3. The inhibitory effects of Met-enkephalin appear to be greater in C 57 than in DBA mice; 4. Different doses of Naltrexone are required to reverse the effects of morphine and Met-enkephalin; 5. Cumulative doses of Met-enkephalin and morphine induce different responses in the vas deferens of C 57 and DBA mice. The results emphasize the usefulness to study analgesic activity in these strains of mice.