An HPLC method with electrochemical detection for the simultaneous measurement of serotonin (5-hydroxytryptamine) and 5-hydroxyindoleacetic acid in primary mesencephalic cell culture is described. The serotonin and 5-hydroxyindoleacetic acid cell content was measured on different days of growth in vitro; after twelve days in culture the amounts of serotonin and 5-hydroxyindoleacetic acid detected were 916.0 +/- 70.2 and 215.8 +/- 15.5 pg per well, respectively. The heterogeneity of neurons in our cultures and their capacity to take up serotonin were assessed by measuring the amounts of exogenous serotonin taken up in the presence of different monoamine uptake inhibitors. This method, sensitive and reliable, can represent a valid alternative to the use of labelled compounds.