Growth characteristics of breast (pectoralis) muscle of male Large White hybrid turkeys were described using the von Bertalanffy model and Huxley's allometric growth analysis, and the cellular maturation process of breast muscle was studied by examining the relationship between the timing of appearance of myosin heavy chain isoforms and circulating levels of thyroid hormones. The inflection points at which the maximum growth rate occurred were estimated to be 112 days for the body weight and 159 days for the breast-muscle weight. The allometric growth coefficient for pectoralis muscle, relative to the whole-body growth, was 1.23 and significantly greater than those of the gastrocnemius, sartorius, latissimus dorsi and extensor digiti muscles during the first 12 weeks posthatch. Myosin was prepared from pectoralis muscle of turkeys at ages ranging from 15 days of incubation (embryos) to 308 days (adult), and embryonic, neonatal and adult isoforms of myosin heavy chain were detected by monoclonal antibodies in the enzyme-linked immunosorbent assay. In turkey embryos, the embryonic myosin heavy chain isoform appeared by 22 days of incubation. The transition from embryonic isoform expression to neonatal isoform expression was noted shortly after hatch, and the transition from neonatal to adult was initiated at 7 days posthatch and was nearly completed by 28 days posthatch. The circulating thyroxine concentration rose and fell markedly around the time of hatch and fluctuated within the range of 11 to 19 ng/ml. The triiodothyronine concentration fluctuated within the range of 5 to 7 ng/ml after hatch and started to fall at 14 days posthatch to reach the adult level of 3 to 5 ng/ml.