Changes in serum estradiol, insulin-like growth factor-1, leuteinizing and testosterone levels, and leuteinizing hormone-stimulated testosterone production per testis in vitro from hatching to adulthood were studied in Korean native chickens of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 21, 24, 28, 32, 44, 52 and 64 weeks (n=13 chickens per group) of age. The changes in the profiles of the levels in the incubation medium of luteinizing hormone-stimulated (100 ng/mL) testosterone secretion per testis in vitro, and the serum LH, testosterone, estradiol, and insulin-like growth factor-I were determined by radioimmunoassay. Serum estradiol levels were not significantly different at week 4 compared to that of 8, 12, 16, 21, 32, :md 44. Significant decreases were observed at weeks 52 and 64. Serum leuteinizing hormone concentrations were not significantly different from 1 week to 12 weeks, increased gradually up to 32 weeks of age, and declined significantly thereafter; the highest value was at 32 weeks, and the lowest value was detected at 2 weeks of age. Serum insulin-like growth factor-I concentrations increased significantly from 1 week to 16 weeks, remained low and unchanged with advancing age. Serum testosterone concentrations were not significantly different at week 1 compared weeks 2, 4, 6, and 8. Significant increases were observed from 10 weeks to 32 weeks of age. Values at weeks 24, 28 and 32 and at weeks 32, 44, 52, and 64 were not significantly different. The highest value was at weeks 28 and the lowest value was detected at weeks 1 week. LH-stimulated testosterone production per testis in vitro increased gradually with age from 1 to 32 weeks and decreased significantly from 44 weeks to 64 weeks of age.