Experiments were carried out on normal, hypophysectomized and gonadectomized Rana temporaria to investigate the possible factors responsible for the induction of encystation in Opalina ranarum. In frogs prevented from copulating, encystation of the parasites took place in female frogs 77 days later than those which had paired, and in the males, 82 days later. Injection of estrone and gonadotropic substances (concentrated or untreated pregnancy urine, chorionic gonadotropin, serum gonadotropin, suspension of frog hypophyses) into normal or hypophysectomized male or female frogs prior to their breeding season induced sexual reproduction in the opalinids, but not when injected at other periods of the year. Injections of testosterone propionate and adrenaline into normal male or female hosts induced cyst formation in the parasites during any time of the year. In gonadectomized frogs, only injections of testosterone propionate were effective in inducing cyst formation in the parasites; adrenaline was ineffective. Testosterone propionate injections, in the doses which caused cyst formation in the parasites, induced precocious spermatozoon formation in testes of the treated hosts. Control injections of saline or distilled water did not produce cyst formation at any time of the year. The exposure of the hosts to artificial light had no observable effect on the parasites. In vitro, gonadotropic hormones were ineffective, but three positive results were obtained with untreated pregnancy urine in cultures prepared just prior to the breeding season. Some cultures treated with estrone or water-soluble extract of the testis, prepared at the same time, produced dividing stages and small forms, but cysts were never produced.