In previous studies we demonstrated the adverse effects of ethyl alcohol in murine head injury. Using this model in this study, effects of prior administration of pentazocine (Talwin) on the survival of mice are reported. Swiss Webster mice were randomly divided into one of three groups. Group I (n= 20) received pentazocine and Group II (n= 20, controls) received saline. Groups I and II were then injured under ether anaesthesia by allowing a weight to fall from a height of 15 cm on the cranium. Ten animals received pentazocine and were anaesthetized but had no head trauma (Group III). In Group II, 85% survived 24 h, and in Group I, 35% survived; all mice in Group III survived 24 h (significance:χ2,p<0.005; Group I versus Groups II and III). Pentazocine increases the lethality of murine head injury.