Equilibrium binding studies in sheep thalamic homogenates indicated that morphine-3-glucuronide (M3G) had an apparent affinity for mu1-opioid binding sites (IC50 = 178 +/- 40 nM, Ki = 116 +/- 25 nM, mean +/- s.e.m., n = 4) similar to that reported by Pasternak and co-workers (1). However, when the chemical purity of M3G was investigated using high-performance-liquid-chromatography (HPLC) with electrochemical detection, it was found to be contaminated with 0.5% (molar basis) of morphine. Reduction of the morphine contamination of M3G to 0.08% resulted in a 7.2-fold decrease in apparent binding affinity (IC50 = 1279 +/- 287 nM, Ki = 766 +/- 30 nM, mean +/- s.e.m., n = 4), indicating that the small percentage of morphine present in the M3G raw material drug is the likely explanation for M3G's apparent binding to mu1-opioid receptors.