We examined the early effects of phrenic nerve denervation on the diaphragm muscle 1, 3, 7 and 14 days after unilateral denervation in rats. In the denervated hemidiaphragms, force frequency curves at 3, 7 and 14 days decreased significantly by 51%, 50% and 38% respectively of the peak tension of the force frequency curves of the diaphragms of rats with sham operation. Twitch tensions increased significantly at 14 days, and contraction times and half relaxation times slowed significantly at 3, 7 and 14 days. The tensions of denervated diaphragms at 5 min during the fatigue runs was significantly increased at 14 days. As determined by histological staining, the mean cross sectional area of fast-twitch fibers (type II) decreased significantly from 2,742 (sham) to 1,599,um2 (14 days), but that of the slow-twitch fibers (type I) did not change significantly during the same period. These findings suggest that, during the first two weeks of denervation, fast twitch fibers (type II) atrophy more rapidly than slow twitch fatigue resistant fibers (type I), as confirmed by the contractile properties and histological findings. twitch kinetics; muscle atrophy; muscle plasticity Hemidiaphragmatic paralysis due to acute phrenic nerve injury is well known as a cause of respiratory muscle failure. Concerning the denervation of skeletal muscles, previous studies have shown that denervation of the skeletal muscles induced muscle fiber atrophy, with an increase in cell membrane resistance (Albuquerque et al. 1971), an enhanced extrajunctional acetylcholine (ACh) sensitivity (Axelsson and Thesleff 1959) and a depolarization of the resting membrane potential in the myocyte (Redfern and Thesleff 1971). Furthermore, recent studies have shown that denervation induces changes in the messenger RNA