Penicillium chrysogenum was cultivated under controlled conditions and at constant specific growth rates regulated by glucose supply. The rate of penicillin synthesis (units/mg dry weight.hr) and pattern of time changes of the synthesizing capacity of the culture depended on the specific growth rate (hr-1). The arrest of penicillin synthesis was induced by the inherent changes of the culture rather than by external factors, e.g. use of substrates, changed limitation, accumulation of inhibitory metabolites, etc.