Recent studies indicate that an N-methyl-D-aspartate (NMDA) receptor system in the rostral medulla is involved in opiate withdrawal. Although NMDA antagonists attenuate naloxone-precipitated opiate withdrawal, they can cause phencyclidine (PCP)like effects that contraindicate clinical use. Because NMDA channels contain sites for the glutamate coagonist, glycine, we assessed the effects of glycinergic agents on naloxone-precipitated opiate withdrawal in rats. The putative antagonist, felbamate (100, 300 mg/kg), attenuated overall withdrawal severity in a dose-related manner and reduced occurrences of chews, teeth chatters, and penile grooming. The partial agonist, D-cycloserine (3, 10 mg/kg), attenuated withdrawal severity, but not in a dose-related manner. Conversely, the low dose of the partial agonist, (+/-)-HA-966 (3, 10 mg/kg), heightened the occurrences of some withdrawal signs. These results support a role for glycine in opiate withdrawal and suggest that these agents, which do not cause PCPlike effects, may be potential treatment for agents for opiate detoxification.