beta-Endorphin was microinjected into rat brain in order to localize central sites associated with some of its pharmacologic effects: namely, analgesia (inhibition of the tail-flick response), catalepsy and changes in body temperature. Microinjections (1 microliter) were made bilaterally under halothane anesthesia and the effects of beta-endorphin were repeatedly assessed at 15- or 30-min intervals for 2 hr. beta-Endorphin produced analgesia and catalepsy when it was injected at low doses (ED50, 1.3 to 2.7 micrograms) into the medial preoptic area, nucleus accumbens, anterior hypothalamus and the periaqueductal gray-4th ventricular spaces. Brain areas of intermediate sensitivities (ED50, 3.7 to 16 micrograms) were the medial thalamus, posterior hypothalamus and areas around the fasciculus retroflexus. The frontal cortex, striatum and lateral areas of the brain were relatively insensitive (ED50 greater than 17 micrograms) to the effects of beta-endorphin on analgesia and catalepsy. beta-Endorphin had complex effects on body temperature. For example, when beta-endorphin was injected into the nucleus accumbens or preoptic area, low doses (1.1--2.1 micrograms) produced hyperthermia; higher doses (8.5 micrograms) produced hypothermia. The brain regions in which low doses of beta-endorphin elicit pharmacologic effects correspond to the anatomic areas in which the endogenous beta-endorphin system is distributed. Similar correspondence to the endogenous enkephalin system was not obtained.