The maximum tetanic tension (Po) generated by a skeletal muscle is determined by its functional cross-sectional area (CSA) and its specific tension (tension/CSA). Measurements of average fiber length (normalized to a sarcomere length of 2.2 micron), muscle mass, and approximate angle of pinnation of muscle fibers within a muscle were taken from 26 different guinea pig hindlimb muscles and were used to calculate CSA. The specific tension was assumed to be 22.5 N X cm-2 and was used to determine the estimated Po of each muscle studied. In a second group of guinea pigs the in situ Po of 11 selected hindlimb muscles and muscle groups were determined. Estimated and measured Po values were found to have a strong linear relationship (r = 0.99) for muscle and muscle groups tested. The specific tension of the soleus, a homogeneously slow-twitch muscle, was shown to be approximately 15.4 N X cm-2 (P less than 0.01). Therefore, in our hands a specific tension value of 22.5 N X cm-2 appears to be a reasonable value for all mixed muscles studied in the guinea pig hindlimb and can be used to estimate their Po.