Food intake (A), average weight change from baseline (B), NPY expression (C) and AgRP (D) expression in mice treated for four days with morphine or placebo pellets or placebo pellets pair-fed to match food intake on the prior day of morphine pellet-mice. Pair-feeding occurred on days 1–4 and morphine mice are shifted by one day so that their food intake is aligned with that of pair-fed mice. Animals were weighed daily and weight change from baseline was calculated. The amount of food consumed per mouse was determined daily. Baseline food intake and weight were measured for two days prior to pellet implantation in placebo- and pair-fed animals and for one day in morphine-treated animals. Hypothalamic NPY and AgRP expression was determined by real-time RT-PCR and corrected for expression of 18S RNA. Food intake, N=8–11 animals, body weight, N=10–12 animals, Gene expression, N=7–14 animals. *, P< 0.05, **, P< 0.01, ***, P< 0.001, ****, P< 0.0001.