The relationship between catecholamine fluorescence and electrical activity of the nerve cell has been investigated in the giant dopamine neuron (GDN) of the left pedal ganglion of the European water snail, Planorbis corneus. Electrical recordings were performed in vitro with intracellular microelectrodes on 35 GDN. The ganglion was frozen to -195 degrees C with the electrode in situ and processed for histochemical microfluorimetry. The intensity of catecholamine fluorescence was measured over different places (42/cell) throughout the cytoplasm of the GDN. In order to investigate the temporal relation between histochemical and electrophysiological parameters, the activity of 21 GDN was changed by administration of nicotine to the bathing solution (10(-5)--10(-7) M). This treatment was followed by depolarization in most of the GDN, with increased firing in two thirds and decreased activity in one third of the spontaneously active cells, whereas hyperpolarization was seen in 4 GDN, accompanied by a decrease in firing. Acetylcholine (10(-5) M) tested on one GDN caused depolarization and increased firing. A signigicant positive correlation was found between the fluorescence intensity of individual GDN and the firing rate of these cells as observed during the last 60 sec or the last 10 sec before freezing. The correlation coefficient dropped markedly when the first rates of the 2nd, 3rd, or 4th and 5th min before freezing were correlated with cellular fluorescence intensity. Intensity was not correlated with the resting membrane potential recorded at the time of freezing. The intensity response to activation was not uniform throughout the cytoplasm of GDN. Neurons with increased mean fluorescence intensity regularly showed small clusters and cristae of intensely fluorescent material surrounded by less fluorescent parts of the cytoplasm. This morphological observation of increased intensity differences between cytoplasmic structures was confirmed by the statistics of the intensity values determined in individual cells. The present observations demonstrate the existence of a relation between cell firing and neuronal dopamine fluorescence at the level of the individual nerve cell. The activity immediately preceding the time of freezing of the cell appears to be most important. Certain components of the cytoplasm appear to react preferentially; their subcellular nature remains to be elucidated. The link between electrical activity and cellular dopamine fluorescene was found to be basically the same in the giatn invertebrate neuron and in the dopamine nerve cells of rat substantia nigra.