The existence of fluid and solid lipid domains in isolated rat-liver plasma membranes was evaluated using the fluorescent fatty acids trans-parinaric and cis-parinaric acid as probe molecules for solid and fluid membrane areas, respectively. The fluorescence probe 1,6-diphenyl-1,3,5-hexatriene indicated that a phase transition was present in the liver plasma membrane between 18 degrees C and 30 degrees C. At intermediate temperatures, cis-parinaric acid, which partitioned approximately equally into fluid and solid lipid areas, detected two lipid domains: the mole fractions of fluid and solid lipid domains at 24 degrees C were 0.32 and 0.68 while the mole fractions of cis-parinaric acid in each domain were 0.34 and 0.66, respectively. The dissociation constant, aqueous to membrane lipid partition coefficient, and bound to free ratio for trans-parinaric acid were 7.0 +/- 0.7 microM, 4.0 +/- 0.6 x 10(6), and 83:17, respectively. The affinity of the membrane for cis-parinaric acid was twofold lower than for trans-parinaric acid. The trans-parinaric acid partitioned preferentially into solid lipid, Ksp/f = 3.30, while the cis-parinaric acid partitioned equally between fluid and solid phases Ksp/f = 0.92. Thus, the data demonstrate the coexistence of fluid and solid domains in rat liver plasma membranes.