Ceramide as central second messenger of the apoptosis-related sphingomyelin signaling pathway is a potential target for the control of cancer. A complex metabolizing network defines cell type and stage-specific final ceramide concentrations. Successful therapeutic control of ceramide levels requires a knowledge of multiple related turnover rates. The metabolism of ceramide and sphingomyelin was studied in keratinocytes under the condition of an unstimulated sphingomyelin signaling pathway. Preparations enriched in plasma membranes contain a neutral Mg(2+)-dependent sphingomyelinase and a Mg(2+)-independent sphingomyelin synthase that vigorously preserve balanced ceramide and sphingomyelin levels. Ceramide regulates neutral sphingomyelinase. Inhibition of sphingomyelin synthase by D609 treatment results in temporary loss of intercelluar contacts and in cellular shrinking. It is ineffective for sustained elevation of ceramide levels. Ceramide phosphorylating and deacylating activities are insignificant. Recently, fatty-acid remodeling in sphingomyelin was reported as likely to counteract the membrane-rigidifying effects of cholesterol. Keratinocytes transfer fluorescence labeled acyl-chains between phosphatidylcholine and sphingomyelin. A transferase of that kind would allow rapid adjustment of local lipid composition in response to acutely changed conditions. In addition, this transferase might have a function in the formation of the epidermal permeability barrier.