Two experiments were conducted that establish an opioid-based, functional-relationship between the taste of sucrose, pain threshold and distress vocalization in isolated 10-day-old albino rats. In the first experiment intraoral infusion of sucrose virtually doubled heat-withdrawal latencies. This elevation was naltrexone (0.5 mg/kg b.wt.) reversible. In the second experiment sucrose infusions caused a rapid and sustained diminution of distress vocalizations in rats totally isolated from dam and siblings. These are the first demonstrations of a causal relationship between a positive affective system and ones mediating pain and stress.