The effect of changes in muscle length on post-tetanic isometric twitch tension potentiation and myosin P-light chain phosphorylation was studied at 23°C in the mouse extensor digitorum longus muscle. The length-tension relationship was determined for the same muscles after a 30 min period of quiescence and between 30 5 and 3 min after a 1.5 s tetanus at Lo. Isometric twitch tension is increased at all muscle lengths after the tetanus; however, the fractional increase in twitch tension rises from 0.2 at L, , to a maximum of 0.3 at 1.2 L,. The fractional increase in twitch tension measured at any fixed muscle length is constant between 30 s and 3 min post-tetanus. P-light chain phosphorylation remains constant between 30 s and 3 min post-tetanus followed by a slow decline to basal values. Under fixed length conditions, there is linear relationship between the relative magnitude of the twitch tension and the extent of P-light chain phosphorylation. Net myosin phosphorylation measured after a 1.5 s tetanus at 1.23 L, is 35% less than that obtained under t h e same conditions at Lo. Thus, contractioninduced phosphorylation of P-light chain decreases with increased muscle length and post-tetanic potentiation at a constant level of P-light chain phosphorylation increases with increasing muscle length. These observations may be consistent with alterations in the sarcoplasmic Ca' ' ion transient as the muscle is lengthened.