We have found that the rate of secretion and turnover of plasma triglycerides is increased in essential fatty acid (EFA)-deficient rats, in comparison with rats fed a diet adequate in linoleate, even though plasma triglyceride levels did not differ significantly between the two groups. Such effects as seen with sucrose or glucose as the dietary carbohydrate. These results stress the need for in vivo kinetic measurements in evaluating dietary effects and re-emphasize that single measurements of tissue concentrations can be equivocal. We plan to determine the extent to which the type and level of dietary fatty acid affect secretion and turnover of plasma triglycerides and/or chylomicrons. We will also test how rapidly triglyceride secretion is restored to normal after feeding different EFA to EFA-deficient rats. We will continue our studies of surface viscosities of phospholipids that vary in acyl chain and/or in headgroups. Surface viscosity was greatly affected by the composition of the polar group of the molecule. Measurements will be made of the effects of sterols other than cholesterol on the surface viscosities of monolayers of saturated phospholipids.