Changes in length of successive 0.5-0.8 mm segments along single muscle fibres of Rana temporaria were recorded during 3 s isometric (fixed fibre ends) tetani at 2.15 and 2.60 micron sarcomere length. The measurements were performed by means of a photo-electric detector system which recorded the distance between opaque markers (ca. 60 microns in width) that were attached to the upper surface of the fibre. The segment length change had an initial rapid phase (1) which coincided with the steep rise of force and a subsequent slow phase (2) which coincided with the upper, rounded portion of the force myogram and the 'plateau' of the tetanus. At 2.15 micron sarcomere length the majority of the central segments (comprising approximately 90% of the fibre) shortened to various degrees during phase 1. A considerable redistribution of length occurred during phase 2 in that some segments shortened at the expense of others which were forcibly stretched. The central region, taken as a whole, shortened by 0.1-0.5% during phase 2. The end segments were consistently found to elongate during phase 1. However, they were able to hold the tension, without further elongation, during phase 2. The pattern of length changes within the central region of the fibre observed at 2.15 micron sarcomere spacing remained largely the same after increasing the sarcomere length to 2.60 micron. However, in contrast to the situation at 2.15 micron sarcomere length there was an over-all (0.4-1.5%) elongation of the central region of the fibre during phase 2 at the great fibre length. This elongation of the central region was associated with marked shortening of the end segments. The sarcomere length of the end segments (s.1.e) was compared to that of the central region of the fibre (s.l.c) at various fibre rest lengths. There was no significant difference between s.l.e and s.l.c when the fibre was just taut, i.e. at approximately 2.1 micron sarcomere length. The following relationship between s.l.e and s.l.c was found to apply for values of s.l.c ranging between 2.2 and 2.7 micron: s.l.e = 0.636 s.l.c + 0.744 (correlation coefficient, 0.93). The possibility was explored that redistribution of sarcomere length along the fibre causes the slow climb of force ('tension creep') that occurs during a tetanus at great (greater than 2.2 micron) sarcomere lengths. Tension creep could be reproduced, after peak force had been attained, during an isometric tetanus by releasing the fibre to shorten within the range 2.6-2.3 micron sarcomere length.(ABSTRACT TRUNCATED AT 400 WORDS)