Synthesis of ethylene in static cultures as well as the effect of endogenous and exogenous ethylene on the synthesis of polygalacturonase by Aspergillus niger were determined. This strain produced maximum ethylene amounts when cultured at 30 degrees C for 3 d. The effect of adding ethylene precursors (citrate-cycle intermediates) on ethylene production was investigated. Best intracellular and extracellular polygalacturonase production was obtained with 2-oxoglutaric, pyruvic and fumaric acids, and with glutamic acid too. Addition of ethylene to the culture medium also increased the synthesis of polygalacturonase, although to a lower degree than when glutamic acid was added.