Bombesin is an oligopeptide which has been implicated as a possible neurotransmitter or neuromodulator in the central nervous system. Central administration of the peptide produces physiological and behavioral responses which suggest a downward shift in the set point of body temperature. These include hypothermia, decreased metabolic rate and increased behavioral responding to escape from radiant heat. Contradictory data also exist. Therefore, a series of experiments was conducted to evaluate further the effects of the central administration of bombesin on behavioral thermoregulation and general locomotor activity. It was found that microinjections of bombesin into the preoptic area of the anterior hypothalamus increased the number of behavioral responses to radiant heat. The increased responding for heat-reinforcement and heat-escape suggested an activation of general locomotor activity. This was confirmed by the finding that centrally-administered bombesin increased the number of responses to escape from radiant heat without altering the duration of exposure to heat. Additionally, bombesin had no effect on the number of heat-escape responses following adrenalectomy. It was concluded that bombesin acts within the hypothalamus to increase general locomotor activity via the sympathetic outflow to the adrenals. This behavioral excitation is apparently expressed as an increase in responding to heat, or escape from heat, independent of thermoregulatory mechanisms. Therefore, the behavioral data do not indicate that bombesin decreases the set point of body temperature.