An autoradiographic procedure is described that allows the quantitative measurement of neurotransmitter receptors by optical density readings. This procedure is a modification of the method of Young and Kuhar (1979a). Frozen brain sections are labeled in vitro with [3H]ligands under conditions that maximize specific binding to neurotransmitter receptors. The labeled sections are then placed against the 3H-sensitive LKB Ultrofilm to produce the autoradiograms. These autoradiograms resemble those produced by [14C]deoxyglucose autoradiography (Sokoloff, 1977) and are suitable for quantitative analysis with a densitometer. Muscarinic cholinergic receptors in rat and zebra finch brain and 5-HT receptors in rat brain were visualized by this method. When the proper combination of ligand concentration and exposure time are used, the method provides quantitative information about the amount and affinity of neurotransmitter receptors in brain sections. This was established by comparisons of densitometric readings with parallel measurements made by scintillation counting of sections.