beta-Endorphin (B-EN) injected intracerebroventricularly in mice produced a rapid onset, dose-dependent antinociceptive effect. The median analgesic dose (AD50) 30 min following administration was found to be 270 ng/mouse (3.7 nmoles/kg). B-EN produced an acute, single-dose tolerance which was characterized by its dose dependence and the time course of its development. Single-dose tolerance development was demonstrable with doses twice or more the AD50. Tolerance was maximal at about 12 h following the priming dose and disappeared within 48 h. Tolerance was accompanied by some degree of physical dependence as noted by signs of naloxone-precipitated withdrawal similar to those elicitable in the morphine-dependent state. Tolerance development to B-EN was blocked by the simultaneous administration of naloxone and also by pretreatment with 0.35 mg/kg actinomycin D or 30 mg/kg cycloheximide 30 min before B-EN. It appears that single-dose tolerance to B-EN was initiated by processes similar to those involved with tolerance resulting from chronic administration of morphine.