The regional and laminar distribution of the serotonin (5-HT) innervation in adult rat cerebral cortex was quantified in radioautographs of semi-thin sections from whole hemisphere slices incubated with tritiated 5-HT and a monoamine oxidase inhibitor. Uptake-labelled axonal varicosities (aggregates of silver grains) were counted with the help of a computerized image analyser, in seven cytoarchitectonic areas of the anterior half of the cortex: Cg3, rostral AID, Cg2, Fr1, Par1, caudal AID and Pir (prepiriform). Since the average diameter of cortical 5-HT varicosities had been previously measured in electron microscope radioautographs, the counts of labelled sites per mm2 of radioautograph could be transformed to numbers of varicosities per mm3 of tissue after a double correction for incomplete detection at the chosen duration of radioautographic exposure and from the thickness of section examined. The mean regional density of cortical 5-HT innervation was thus evaluated at 5.8 million varicosities per mm3 of tissue, ranging from 4.4 (Cg2) to 7.1 million (AIDr). Cg2, Par1 and Fr1 showed significantly lower regional densities than Cg3, AIDc, Pir and AIDr. The highest laminar density was that of layer I in all regions except Pir, but each region had a distinct pattern of 5-HT innervation. In Par1, for example, there was a second band of predilection amounting to 5.3 million varicosities per mm3 at the height of layer Va. On the basis of these figures, it was possible to extrapolate the average number of cortical 5-HT varicosities per midbrain raphe nerve cell body of origin (500,000 at least), their average number per cortical target neuron (145-230), their incidence among all axon terminals in cortex (1/200), their mean endogenous amine content (0.045 fg), and their mean number per varicosity of recognition sites for reuptake blockers such as paroxetine, citalopram or cyanoimipramine (5000-6200). Such quantitative data and further correlations between 5-HT innervation density and other measurable aspects of cortical structure and function should help to elucidate the role(s) of 5-HT in this part of the brain.