Plasmalemmal folds and caveolae were investigated by qualitative and quantitative analysis of electron micrographs obtained by freeze fracture and transmission electron microscopy (TEM) of rabbit right ventricular papillary muscles whose mean sarcomere lengths ranged from 1.64 to 2.28 fun. In passively extended muscles, folds were observed at sarcomere lengths of 2.3 /an and could be shown by extrapolation to become completely extended at a maximum sarcomere length of 2.8 μn. It was concluded that the plasmalemma does not contribute to resting tension in the physiological range of sarcomere lengths. Caveolae are present in both the external plasmalemmal envelope and T-tubular plasmalemma. They show no preferential distribution with respect to underlying myofibrillar striations or membrane folds and are nearly devoid of membrane particles in freeze-fractured material. The surface density of caveolar necks (4.0/pm:apparent plasmalemmal area) is only 16-20% of that reported for frog skeletal muscle. Caveolae augment plasmalemmal area by 21-32%, assuming two or three caveolae per neck, respectively. Caveolar membrane does not serve as a reservoir of membrane to be recruited into external plasmalemma, at least over the physiological range of sarcomere lengths. In heart muscle they do not account for the T-tubular access resistance, and their function in this tissue remains unknown. Circ Res 46: 244-255, 1980