To compare cerebral protein metabolism rates in vivo, protein synthesis rates of three organs of five vertebrate species were measured after a single i.p. injection of a flooding dose of [1-14C]valine. In muscle, brain, and liver, the respective average protein synthesis rates, expressed as percent of total protein-bound valine replaced per hour, that is, percent synthesis per hour, in goldfish at 22 degrees C body temperature, were 0.07, 0.23, and 0.57%; in the bullfrog at 20 degrees C, 0.06, 0.18, and 0.55%; in the white Leghorn chicken at 39 degrees C, 0.24, 0.70, and 2.17%; and in the mouse at 38 degrees C, 0.22, 0.65, and 2.0%. In the Tokay lizard at different body temperatures, the synthesis rates were 0.04, 0.13, and 0.43% at 26 degrees C; 0.05, 0.20, and 0.63% at 32 degrees C; and 0.07, 0.27, and 0.81% at 38 degrees C. The results demonstrate differences in protein synthesis rates in organs of the various species examined. The differences among the species seem to be due, to a major extent, to differences in body temperature; rates in lizard are below those in other species at temperatures tried. Protein synthesis rates in brain in all species are almost three times lower than those in liver and almost three times higher than those in muscle.