Succinate is an interesting chemical for industries producing food and pharmaceutical products, surfactants, detergents and biodegradable plastics. Succinate is produced mainly by a mixed-acid fermentation process using anaerobically growing bacteria. However, succinate excretion is also widespread among fungi. In this article we report results on the intracellular concentration and the excretion of succinate by Penicillium simplicissimum under aerobic and anaerobic conditions. The intracellular concentration of succinate increased slightly with the specific growth rate and strongly if the respiratory chain was inhibited by sodium azide or anaerobic conditions (N(2)). A strong increase of succinate excretion was observed if the respiratory chain was inhibited. It is suggested that succinate synthesis under functional (sodium azide) or environmental (N(2)) anaerobic conditions occurs via the reductive part of the tricarboxylic acid cycle. Succinate is then excreted because the oxidative part of the tricarboxylic acid cycle is inactive. A possible role of succinate synthesis in the regeneration of NAD ('fumarate respiration') is discussed.