A μ-opioid receptor antagonist (Ki = 0.47 nM); selective over κ- and δ-opioid receptors (Kis= 100 nM and 12 nM, respectively); inhibits μ-opioid receptor mediated GTP binding to CHO cell membranes (IC50 = 1.7 nM); inhibits morphine-induced slowing of colorectal transit in mice (ED50 = 0.41 mg/kg, p.o.); antagonizes morphine-but not apraclonidine-induced inhibition of GI transit in rats (0.3 and 1 mg/kg, p.o.)