The biological effects of pure ICSH in [male] and [female] immature rats, both normal and hypophysectomized, was described. This hormone, as well as crude pituitary extracts, when assayed in the Long-Evans strain of rats according to the methods proposed for "LH" proved very ineffective in increasing [male] accessory wts. or in causing ovarian wt. augmentation in combination with FSH. Crude pituitary extracts as well as pure ICSH caused great seminal vesicle wt. increases in another rat strain while preps. rich in "LH" failed to do so in Long-Evans rats. Assays of various urine and serum gonadotrophins in the Long-Evans strain of rats showed that all these preps. (with the exception of the gonadotrophin from menopause urine) produce at low doses the effects characteristic for "LH," i.e., pronounced increase in the wts. of accessories in [male] rats and in ovarian wts. in combination with FSH in [female][female]. This was true not only for gonadotrophins of chorionic origin but also for those which were in all probability secreted by the pituitary. A working hypothesis was proposed to explain the difference in the gonadotrophic effects of pituitary extracts in different strains of rats.