Area under the curve (AUC) for kaolin intake from time 0 to 120 hr. Naloxone significantly reduced kaolin intake induced by ritonavir (P < 0.01 compared to vehicle). Naloxone 0.3 mg/kg alone did not affect kaolin intake. Data are presented as mean ± SEM. RIT, ritonavir 20 mg/kg; NLX0.1, naloxone 0.1 mg/kg; NLX0.3, naloxone 0.3 mg/kg.