Experiments were performed in rats expecting and/or having received rewards to see if naloxone-antagonizable changes occur in behavior. It was found that rats expecting to receive candy display a naloxone-blockable increase in nociceptive thresholds and a naloxone-sensitive increase in rearing. Similarly, water-deprived rats expecting to receive water show a naloxone-blockable increase in rearing, whereas thirsty animals not expecting water show no changes in nociception or activity. Naloxone was also found to reduce the consumption of a highly palatable food and to diminish the performance of rats trained to wait in one place to receive candy. The latter indicates that the naloxone effect upon goal seeking is not dependent upon a decrease in general activity. Experiments in highly morphine-tolerant rats maintained on relatively constant morphine concentrations showed that these rats drink chocolate milk in the same way as placebo-treated animals. The same rats were found to fail to decrease their intake to the same extent as placebo-controls when water is substituted for the first time for chocolate milk given regularly every day. This points to the possibility that a negative action of endorphins, that is a decrease in release, decreasing consumption, upon a reduction in the palatability of an expected food, might be absent in highly tolerant animals. It is suggested that endogenous opiate(s) regulate mood as defined by the level of goal-seeking behavior, sensitivity to noxious stimuli and general activity.