The relationships among body weight, body composition, adipose tissue cellularity and the onset of sexual maturity were studied in Japanese quail (Coturnix coturnix japonica). In an initial experiment, body composition and sexual maturity were examined in response to photoperiodic and hormonal manipulations. Attempts were made to modify body composition, and thereby age at sexual maturity, by feeding diets containing PTU (4-phenyl-2 thiouracil), by imposing a daily photoperiod (IP) of four hours, by the subcutaneous implantation of estrogen to females and testosterone to males, and by the intramuscular injection of turkey growth hormone and anti-sera to turkey growth hormone. The quail were sacrificed at 63 days of age and body composition was examined. Quail subjected to the IP treatment failed to enter sexual maturity by 63 days of age, while about 50% of the controls receiving a photoperiod of 14 hours were sexually mature at this age. The failure of these quail to mature was associated with a reduction in body fat. The effects of the other treatments on sexual maturity, however, were not consistently associated with differences in body weight and/or body composition. In the second phase of this study, adipocyte hyperplasia was examined in relation to sexual maturity. Total DNA concentrations in the stromal and lipocyte fractions of collagenase-digested abdominal fat depots were determined on a chronological age basis from 28 to 240 days of age. In both males and females, significant increases in abdominal fat weights at ages beyond the age at sexual maturity were associated with increased DNA concentrations in abdominal fat adipose tissue. These results suggested that mature Japanese quail were capable of hyperplastic fat deposition. In the final phase of this study, the incorporation of methyl-³H-thymidine into DNA of lipid and non-lipid fractions of collagenase-digested abdominal fat from both ad libitum and restricted-fed (70% of ad libitum intake) quail was examined. Regardless of the feeding regime and stage of maturity, substantial radioactivity was recovered from both the stromal and lipocyte fractions when the quail were examined 24 hours after the administration of tritiated thymidine. When quail were examined five days after the administration of tritiated thymidine, an apparent migration of radioactivity from stromal to lipocyte DNA occurred. It was concluded that this pattern represented mitotic activity of adipocyte progenitor cells located in non-lipid fractions of adipose tissue, and the subsequent maturation of these cells into lipocytes as they accumulated lipid. The hypothesis that adipocyte progenitors reside in both the stromal and lipocyte fraction, however, was not disproven. Nevertheless, the similarity of results obtained prior to, at, and subsequent to the onset of lay indicated that adipocyte hyperplasia contributes to increases in adipose tissue mass in mature Japanese quail. / Ph. D.