In contrast to the rapid development of tolerance to morphine in CD-1 mice, tolerance is not seen in 129/SvEv mice implanted with morphine pellets or given daily morphine injections for 5 days. Similarly, the progressive and complete loss of analgesia in CD-1 mice seen with repeated dosing of the delta ligand [d-Pen²,d-Pen⁵]enkephalin is not observed in 129/SvEv mice. In contrast, tolerance develops normally to both the kappa ₁ drug U50,488H and the kappa ₃ agent naloxone benzoylhdrazone. N-methyl-d-aspartate (NMDA) given alone attenuates morphine analgesia in CD-1 mice and accelerates the development of tolerance in CD-1 mice when given daily with morphine. In contrast, NMDA has no significant effect in the 129/SvEv mice in either paradigm. Activation of NMDA receptors can lead to the production of nitric oxide, which also is involved with morphine tolerance. Sodium nitroprusside andl-arginine increase nitric oxide levels and decrease morphine analgesia in both the control CD-1 and 129/SvEv mice. Thus, the defect in the NMDA/nitric oxide cascade responsible for the loss of morphine tolerance in the 129/SvEv mice rests at the level of the NMDA receptor itself or in the steps up to the activation of nitric oxide synthase.