No examination to date has been made of apoptosis during vertebrate muscle development. The authors recently reported programmed muscle cell death to be important in tail degeneration as well as in the larval-to-adult conversion of the dorsal body muscles of Xenopus laevis during metamorphosis [30]. In the present study, we examined programmed cell death (PCD) of the dorsal body and tail muscle morphologically and biochemically, with special attention to whether apoptotic processes, such as chromatin fragmentation and apoptotic body-formation actually occur, and whether triiodothyronine (T3) induces such processes. Light microscopic observation indicated muscle fibers break down into short fragments (sarcolytes or muscle apoptotic bodies) during the metamorphic climax, not only in tail but also in larval-type fibers of dorsal body muscles. Apoptotic bodies first appeared near the base of the tail in early climax (stage 59) when the T3 level is quite high, and thereafter expanded in an anterior direction in the dorsal body and posteriorly in the tail. The ratio of apoptotic area to total muscle area became maximum (10%-30% in dorsal body muscles and 50% in the tail) at the climax (stages 63-64). During these stages, genomic DNA fragmented into oligonucleosome-sized units (200 bp, 400 bp, 600 bp ...) in both body and tail muscles. To confirm whether this chromatin fragmentation is associated with apoptotic bodies, in situ DNA nick end labeling (TUNEL) was applied to sections of the dorsal body and tail muscles. Labeled muscle nuclei could be found only in muscle apoptotic bodies but not in intact muscle fibers, indicating DNA fragmentation was associated with cell fragmentation during metamorphosis. It thus follows that morphological (apoptotic body formation) and biochemical (fragmentation of chromatin) processes occurring during PCD of dorsal body and tail muscles are identical. To determine whether T3 regulates programmed muscle cell death, the effects of T3 on DNA ladder formation were examined in tails cultured in vitro. The oligonucleosomal DNA ladder was found to form only in tails incubated with T3, thus showing T3 to induce programmed muscle cell death without interaction with other endocrine organs during metamorphosis.