The effects of feeding a diet containingtransfatty acids on the fatty acid composition of plasma membrane phospholipids, fluorescence polarization of diphenylhexatriene (DPH) and adenylate cyclase activity in the exorbital lacrimal glands of rats were studied. Three groups of male, weanling rats were fed semipurified diets containing 20% corn oil (CO), 20% partially hydrogenated soybean oil (PHSBO) (a source oftransfatty acids) and 19% PHSBO + 1% CO. Plasma membranes of the lacrimal glands from rats fed 20% PHSBO showed higher adenylate cyclase activity and lower fluidity as shown by a lower double bond index of the fatty acids of their phospholipids and higher fluorescence polarization of DPH. When 1% CO was included with the diet containing PHSBO, the adenylate cyclase activity and membrane fluidity tended to be normal. The results suggest that feeding of a diet containingtransfatty acids in the absence of sufficient linoleic acid (18:2) can result in a decrease in membrane fluidity and an increase in adenylate cyclase activity in the lacrimal glands.