"Dark" cell formation has been studies in the adrenal medulla with combined light- and electron-microscopic methods. Correlation between light- and electron-microscopic appearance has been demonstrated. It has been pointed out that the formation of "dark" cell artefact can be prevented during immersion fixation and can be produced also during perfusion fixation by appropriate physical conditions. "Dark" cell formation was elicited by prefixation cooling and/or by increased tissular pressure. The nature of the underlying physicochemical change was interpreted as a uniformly distributed sol-gel transition of cytoplasmic macromolecules. The formation of cytoplasmic gel structures can be considered an endothermic process which involves volume increase (+ deltaV) - as may be inferred by their susceptibility to solation by high pressure and low temperature.