Experimentally naive rats were implanted with a cannula in the lateral ventricle of the brain and were allowed to self-administer heroin in doses of 0.125, 0.25, 0.5, 1 or 2 micrograms/infusion or placebo for five daily sessions of 3 h. The number of self-injections was related to the unit dose in an inverted U-shaped manner: the 0.5 microgram/infusion dose induced the highest infusion rate. The total heroin intake was proportionally related to the unit dose. A hot-plate test performed immediately after the fourth session revealed no analgesic effects in any of the groups that self-administered heroin. A naloxone (10 mg/kg) challenge given immediately after the fifth session induced very mild withdrawal signs only in the group that administered the highest dose of heroin. The results indicate that rats readily acquire intraventricular heroin self-administration behaviour and suggest that the rewarding effects of heroin can be dissociated from its analgesic and physical dependence-inducing effects.