We have used light microscopic autoradiography to localize differentially Type 1 and 2 benzodiazepine receptors using pharmacologic and detergent treatment techniques. The triazolopyridazine CL218872 has preferential affinity for Type 1 receptors from which it displaces [3H]flunitrazepam selectivity. In biochemical experiments [3H]flunitrazepam binding sites with pharmacologic properties of Type 2 receptors are preferentially solubilized from brain membranes by detergents. We have now treated rat brain slices either with CL218872 or 2% sodium cholate and evaluated the autoradiographic distribution of [3H]flunitrazepam-labeled receptors. Whether calculated from the drug or detergent treated preparations, the relative densities of apparent Type 1 and 2 receptors are the same. Thus differential sensitivity to determine solubilization distinguishes the same two populations of receptors as Type 1 and 2 specific drugs.