In guinea pig brain membranes 7-benzylidenenaltrexone (BNTX) possesses 100-fold greater affinity (Ki = 0.1 nM) for [3H]DPDPE [3H][D-Pen2,D-Pen5]enkephalin) binding sites (delta 1) relative to those of [3H]DSLET ([3H][D-Ser2,Leu5]enkephalin-Thr6) (delta 2). The ED50 dose ratio (tail flick) in mice for the antagonism of DPDPE-induced antinociception of BNTX (6.3 pmol i.c.v.) was 7.2, whereas for DSLET, morphine and U69593 it was not significantly different from unity. The fact that there was no correlation of the binding or in vivo data for BNTX with antagonist potency in smooth muscle preparations suggests that the in vitro pharmacologic activity is mediated by delta-opioid subtypes that are different from those in the brain.