Representative species of the families Pleuronectidae, Bothidae and Soleidae were studied throughout the year. Changes are described which were seen in the ovaries, and photographs of cross sections of the tissue are shown. The progress of egg release was noted, and in some cases correlated studies were made of gonadal changes in male. The period of egg release was prolonged over 1 one half -2 months for plaice and turbot, and for 6 months for sole. Individual fish showed these extended periods of laying, and the actual months of the year differed for each species. The massive mobilisation of reserves which necessarily accompanied the release of large numbers of eggs was reflected by turbot, in sperm production. In addition, the increase in gonadosomatic activity was preceded by an increased hepatosomatic activity. These fluctuations were not evident in sole. It is suggested that the sole would be less suited to an experimental breeding system than would be turbot or plaice.