Repeated immunization of rats with beta-endorphin-bovine albumin conjugate (75:7.5 g) mixed with Freund's adjuvant (1:1) induced a significant decrease in beta-endorphin content in the pituitary body and hypothalamus. The immunized rats showed suppressed antinociceptive reaction to morphine (5 mg/kg, i.p.) and in unavoidable foot shock. Cold swimming stress did not influence the pain reactions, as compared to the control group. The results indicate that mechanisms of different types of analgesia involve selective neurochemical systems.