In Experiment 1, adult male rats were given eight 30-min exposures to one of two distinct environments. Control animals received either four exposures to each environment or were not exposed to either environment. When given free-choice access to both environments simultaneously, animals spent significantly more time in the novel environment relative to the familiar environment. In these same animals, horizontal and vertical activity rates were lower in the novel environment than in the familiar environment. In Experiments 2-5, animals were assessed for novelty preference behavior under the influence of either morphine (0, 0.1, 0.3, 1.0 or 3.0 mg/kg), naltrexone (0, 0.1, 0.3 or 1.0 mg/kg), amphetamine (0, 0.1, 0.3 or 1.0 mg/kg) or haloperidol (0, 0.03, 0.1, 0.3 or 1.0 mg/kg). Haloperidol produced a dose-dependent disruption in novelty preference behavior, while all other drugs tested were without effect. Haloperidol also disrupted the novelty-induced decrease in horizontal and vertical activity rates. These results suggest that haloperidol blocks the reinforcing and locomotor-depressant effects of a novel environment in a free-choice preference test.