In vertebrate striated muscle the titin/connectin molecule spans half a sarcomere, and in the I-band forms an elastic filament connecting the Z-line with the end of the thick filaments. The only part of the elastic filament that has been described in intact rest length muscle is a short extension to the thick filament observed in freeze-fractured cardiac muscle which has similarities to the end-filament of negatively stained isolated thick filaments. We report here further observations made in sections of rabbit psoas muscle. In very thin longitudinal sections thin extensions to the thick filaments some 0.11 mu m long and 5-6 nm in diameter are seen. Transverse sections show that each thick filament has such an extension. Nothing similar is seen further into the I-band or at the Z-line. The common features of this structure in both cardiac and skeletal muscle suggest that it corresponds to a common sequence in their titins. Such a sequence is to be found in the 22 tandem Ig domains near the A/I junction. Taken together with other information about the arrangement of domains in this part of the sarcomere, this leads to a calculated length for the end extension of 104 nm. The length of the extension does not vary with sarcomere length between 2.2 and 3.0 mu m and therefore it corresponds to an inelastic region of I-band thin over the physiological range. Each extension probably comprises part of three to six titin molecules depending on the complement of titin in the thick filament, as previously suggested. A polymer formed from several strands of Ig domains would make for a relatively rigid structure which would resist folding or stretching when subjected to the small passive forces which pertain over the physiological range of sarcomere lengths. The relationship of the N2-line with the end-filament has also been studied. The N2-line position varies with sarcomere length in an elastic manner. Only at short sarcomere lengths does the end of the end-filament coincide with the position of the N2-line. Taking into account recent work on the elasticity of titin in the I-band we conclude that the N2-line corresponds to part of the elastic PEVK region of titin and not the region of thin sequence designated N2. (C) 1997 Academic Press.