1 Control experiments were carried out by homogenizing rat brain at 0 degrees C with sucrose containing various concentrations of [3H]-etorphine. Subcellular fractionation of this homogenate showed that the distribution of the labelled drug amongst the primary fractions was dependent on the concentration of etorphine in the homogenate. 2 Rats were injected intravenously with 0.2 and 20 microgram/kg of [3H]-etorphine. The brains were homogenized and fractionated in sucrose containing 4.2 x 10(-5) M unlabelled etorphine in order to control redistribution artifacts. Different distribution profiles in the subcellular fractions were observed at these two dose levels. 3 Concurrent administration of either cyprenorphine or naloxone with intravenous etorphine, caused a shift of the labelled drug from the P3 fraction to the supernatant fraction. 4 The subcellular distribution of intravenously administered [3H]-etorphine was also studied by homogenizing brains in etorphine-free sucrose, and sucrose containing either levorphanol or dextrorphan. From these experiments it was concluded that the P3 microsomal fraction is a major site to which in vivo etorphine is stereospecifically bound in the rat brain.