Observations on the increase of ovarian weight in the female humpback whale cover stages from late foetal life until after the attainment of sexual maturity. Variations in ovary weight during different phases of the reproductive cycle are recorded. Graafian follicles up to 0.13 mm in diameter have been found in the foetal ovary just prior to birth. These initially develop until from 11 to 29 mm (median value 20 mm) in diameter when the female reaches the maturing length range. At sexual maturity one of these follicles enlarges, exceeding 30 mm in diameter before rupture. After ovulation a corpus luteum is rapidly formed and if conception has taken place, this gland enlarges and remains active until parturition. After parturition there is rapid resorption of the corpus luteum to form a corpus albicans. Variations in the sizes and weights of functional corpora lutea are discussed. There is considerable reduction in the diameters of Graafian follicles in late pregnancy to a median value of 6 mm. After parturition follicles enlarge so that in the ovaries of females in late lactation the largest follicles once more range from 11 to 29 mm in diameter with a median value of 20 mm. In an ovulatory period of from June to November the maximum frequency of ovulations is in late July, though considerable numbers occur in August and September. On the availabIe evidence most female humpback whales ovulate only once in their ovulatory period, a few ovulate twice, and extremely few three times.