The formaldehyde induced fluorescence in perikarya localized in the midbrain rephe nuclei was investigated using the Falck-Hillarp technique in combination with qualitative (spectral analysis) and quantitative microspectorfluorimetry. The spectral evidence obtained after various pharmacological and lesion experiments with the neurotoxic compounds 5,6-dihydroxytryptamine and 5,7-dihydroxytryptamine, strongly favours the view that the vast majority of the perikarya in the cell groups B-7, B-8 and B-9 (according to Dahlström and Fuxe) are 5-hydroxytryptamine neurons, defined as structures capable of synthesizing, metabolizing, and storing 5-hydroxytryptamine. The spectral data indicate that the 5-hydroxytryptamine neurons might contain in addition to 5-hydroxytryptamine another indolealkylamine, possibly tryptamine, in low concentrations. The perikarya were shown to be able to take up and accumulate exogenously administered 6-hydroxytryptamine provided that monoamine oxidase was inhibited. Quantitative microfluorimetric analysis disclosed that the tryptophan hydroxylase inhibitor p-chlorophenylalanine was unable to block effectively this enzyme in the 5-hydroxytryptamine perikarya, although acutely a partial blockade was observed. The 5-hydroxytryptamineerogenously to the action of p-chlorophenylalanine and this might be associated with different states of neuronal activity. The difference in potency of p-chlorophenylalanine as regards tryptophan hydroxylase inhibition in perikarya and in nerve terminals may be related to different properties of tryptophan hydroxylase in various parts of the neuron and/or to a high turnover of the enzyme in the perikarya.