Apparent insensitivity to pain, ritualistic patterns of behavior, and improvement in symptoms after administration of opiate receptor blockers implicated the endogenous opioid system in the initiation and maintenance of SIB. This study was designed to determine whether plasma levels of proopiomelanocortin (POMC)-derived peptides, beta-endorphin-like activity (beta E), ACTH, and adrenal cortisol immediately after an episode of SIB predicted subsequent response to an opiate blocker. Blood samples were collected from 10 patients with mental retardation within minutes of a self-injuring act and during an SIB-free control period. On another day, morning and afternoon samples were collected at least one week apart from the other samples. Effects on SIB of naltrexone hydrochloride (NTX) were examined in a double-blind, placebo-controlled crossover study. After an SIB episode, beta E, but not ACTH, was elevated compared with morning levels, p < .003. Patients with increased plasma levels of beta E after SIB had the most positive response to 2 mg/kg NTX, p < .03. Results suggest that changes in the hypothalamic-pituitary-adrenal axis after SIB may predict differences in individual patient response to opiate blockers.