A long term treatment with the delta-selective opiate antagonist NN-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH (ICI 154129) produces an increase in the number of delta-opiate binding sites, whereas the same treatment with the non selective opiate antagonist naloxone results in an enhancement of both mu- and delta-binding sites. This biochemical effect in naloxone-pretreated mice is paralleled by a more pronounced increase in locomotor activity induced by a challenge dose of morphine. In contrast, no difference in the effect of morphine was seen in ICI 154129-pretreated mice with respect to control. These data suggest that the locomotor response to morphine in C57 mice is not mediated through delta-opiate receptors.