The ultrastructure of the contractile apparatus was observed in muscles maintained in excessive extension, i.e. in conditions in which an increase takes place in the number of sarcomeres. Rat leg muscles (soleus, extensor digitorum longus and gastrocnemius) were studied, at variable time intervals in the range 3-7 days. Several irregularities were found in the contractile structure. The most frequent were the variability of sarcomere length, the appearance of 'extra' sarcomeres, irregularities of the Z-line (including Z-band 'streaming') and A-bands of abnormal length. The character of these irregularities depended on the muscle fibre type. Variations of the Z-line were seen mostly within continuously working fibres, especially slow ones, while anomalies in the size of the A-band and variability of the sarcomere length were more pronounced in fast fibres. All these irregularities appearing in the muscles maintained in excessive extension were also occasionally found in control muscles. The reasons for these contractile structure irregularities, and their possible significance for contractile structure reorganization, are discussed.