The contractile performance of isolated muscle preparations from the left ventricle of rats receiving a 3% casein protein deficient diet for two to three weeks was compared with the performance of muscle preparations from animals receiving a normal diet. The right and left ventricular weights from protein deficient animals were significantly decreased, but left ventricle/body weight and right ventricular/body weight ratios did not differ from control animals. The performance of the isolated muscle preparations revealed no differences in the following parameters measured at the apex of the length tension curve: peak active tension, resting tension, maximum rate of tension development and time to peak tension. RT1/2, an index of relaxation, and maximum rate of tension decline indicated relaxation rate was increased in preparations from protein deficient animals. We conclude that, while a protein deficient diet of two to three weeks duration results in proportionate decreases in body and cardiac chamber weight in comparison to control animals, no evidence for impaired contractile activity is seen.