Step changes in the gas environment and temperature during membrane culture of Rhizopus oligosporus were made to mimic those changes which arise during solid-state fermentation due to mass and heat transfer limitations. A decrease of O-2 concentration from 21% to 0.5% did not alter protease production by R. oligosporus but retarded amyloglucosidase production. An upshift from 37 degrees C to 50 degrees C decreased the activities of both enzymes, with the effect on protease being due to enzyme deactivation rather than a decrease in production.