A study was conducted to determine and compare levels of LH, FSH and total gonadotropins in pituitaries of orchiectomized and cryptorchid animals at various time intervals after surgery. In preparation for this study, a comparison of 3 different techniques for determination of rat pituitary gonadotropins was made. The ovarian weight for "total" gonadotropins. The Steelman-Pohley augmentation assay for FSH, and the OAAD [ovarian ascorbic acid depletion] method for LH were assessed. All 3 methods were found to be adequately sensitive and precise. The LH content of normal adult rat pituitary, in terms of NIH-LH-S-1, was equivalent to 10.7 [mu]g/pituitary. The FSH levels, in terms of NIH-FSH-S-1, were 199 [mu]g/pituitary. In orchiectomized animals, the FSH levels dropped below the normal levels 1 week after surgery; at 2 weeks they returned to normal and then gradually showed an increase up to the period of 1 yr. The LH levels showed an increase 1 week after orchiectomy and then a progressive rise until 1 yr. In cryptorchid animals, 1 week after surgery, there was a significant rise in FSH levels. This rise was progressive and the levels remained elevated up to 1 yr after surgery. Similarly, the LH levels showed a rise 1 week after surgery and remained elevated up to 1 yr after surgery. In both orchiectomized and cryptorchid animals, a temporary drop in LH levels was observed 8 weeks after surgery. This drop needs to be substantiated before it can be interpreted. A comparison of the patterns of changes in gonadotropin levels between orchiectomized and cryptorchid animals led the authors to suggest the hypothesis that the Leydig cell secretions influence the release of FSH from the pituitary while germinal epithelium influences the production of this hormone.