Systemic morphine exposure disrupts both maternal behavior (MB) and postpartum aggression, possibly through alterations of olfactory preferences [29]. In the current studies, adult female rats were timed mated and implanted with a unilateral cannula in the lateral ventricle. On day 5 or 6 of lactation, the females were infused with either morphine (2.0 micrograms) or saline (5 microliters) (Experiment 1); or, they were infused with 2.0 micrograms morphine and saline or morphine plus 0.5 micrograms naloxone (Experiment 2). One hour later, they were exposed to bedding soiled by pups, or to clean bedding. The amount of time spent investigating the two bedding types was compared. Morphine produced an aversion to the odor of pups, relative to the saline condition, wherein the females expressed a preference for the odor of pups. Naloxone reversed the effect of the morphine, restoring the preference for pup odors in the females. Thus, morphine disruption of MB may be due to central action on olfactory sensory mechanisms.