In the rabbit, radical surgical thyroidectomy leads to increased ovarian weight and a tendency to development of polycystic ovaries. This process is preceded by an increase in the ovarian content of acid mucopolysaccharides. (1) 10 days after thyroidectomy the ovaries showed an increased number of growing follicles, a highly increased quantity of non-sulphated metachromatic substance, and an increased quantity of hexosamine. (2) 20 days after thyroidectomy there was, moreover, an increased number of Graafian follicles, an increased synthesis of 35S-containing mucopolysaccharides, and a significant increase in ovarian weight. (3) 30 days after thyroidectomy polycystic ovaries were found in 4 out of 10 rabbits. The number of growing follicles as well as of Graafian follicles was increased. In addition, there were large, atretic follicles without ovum, with a fainter metachromasia, and only little incorporation of 35s. The hexosamine content as well as the synthesis of 35S-containing mucopolysaccharide was also increased in this group. (4) 50 days after thyroidectomy 5 out of 10 rabbits had enormously enlarged, polycystic ovaries. The synthesis of 35s-containing mucopolysaccharides was reduced, while the hexosamine content was still increased.