The effects of various concentrations of 6-hydroxydopamine (6-OHDA) on rat adrenocortical cells in tissue culture were studied with phase contrast and electron microscopy. With 40 mg/l of 6-OHDA the first signs of alteration as revealed by microcinematography appeared in isolated cortical cells as early as 15 min after addition of the drug. There was a cessation of movement of cell organelles and an immobilisation of membrane undulations followed by the development of dark inclusion bodies. The cells underwent increasing shrinkage and collapsed by 1 1/2 h. Chromaffin cells were not affected until 45 min after exposure to the drug and neurons were the most resistant population. However 6 1/2 h after application of the drug most cells in the culture were dead. 6-OHDA applied in different doses and to adrenal explants did not alter the sequence of events. Ultrastructurally cortex cells underwent damage alone two lines: they either showed lytic changes or developed various types of dense bodies before reaching the lytic stage. Treatment of cortical cells with 40 mg/l 5-or 6-OHDA followed by exposure to buffered 2% glyoxylic acid and heat did not produce a fluorescence within the cells. Microspectrofluorimetry on amine models with noradrenaline, 5- and 6-OHDA revealed that neither 5-nor 6-OHDA are capable to form a fluorophore with glyoxylic acid.