Active opioid receptors were solubilized from frog (Rana esculenta) brain membranes using 1% digitonin. The solubilized preparation was sedimented in sucrose density gradient and applied to Sepharose-6B column. In the ultracentrifugation experiments, two distinct molecular forms of the opioid receptors were observed with apparent s20, w values of 15.7 and 10.8 S. The estimated molecular weights were 470 and 180 kD. The Stokes radii of the two separate forms were determined by gel filtration and found to be 71 and 42 A. The corresponding molecular weights were 500 and 140 kD indicating a good correlation with data obtained from the sedimentation experiments.