A uterine weight response may potentially provide a plausible qualitative method for detection of "asialo" follicle stimulating hormone (FSH). Asialo FSH is a biologically inert species of FSH when assayed for ovarian weight response by the hCG augmentation method. Numerous experiments have been performed under a uniform set of conditions for normalization of ovarian and uterine weight responses. The influence of several hormones and 2 injection routes on the gonadal responses to FSH have been assessed. Immature female, 24--25 day old Sprague-Dawley rats were injected subcutaneously (sc) or intraperitoneally (ip) twice daily for 3 days. On the 4th day the rats were sacrificed, the ovaries and uterus were removed, cleaned and weighed. Uterine weights were obtained before and after removal of the fluid. NIH-FSH-S9 and its asialo derivative prepared by neuraminidase digestion were the species of follicle-stimulating hormone (FSH) used in all experiments. Addition of either 21 IU prolactin or 0.05 mg histone to FSH did not significantly alter the ovarian weight response augmented by hCG (P = 0.2). Addition of histone to 90 and 180 microgram FSH elicited neither a uterine weight response when injected sc nor an ovarian weight change when injected ip. Both routes of injection of the 2 dose levels of FSH had little effect on the uterine weight. In marked contrast, 90 and 180 microgram asialo FSH injected either sc or ip gave increased uterine weight responses in the presence of hCG. On further examination the uterine weight increase in response to asialo FSH injections proved to be statistically significant (sc. P = 0.02; ip, P = 0.05). Use of the uterine weight response might be exclusively proposed as a qualitative method for detecting asialo FSH. Such biologically inactive species, which elicit no increased ovarian weight response in the hCG augmentation assay, are frequently generated in the preparation of FSH. Determination of the uterine weights may provide meaningful use of auxiliary data from the hCG ovarian augmentation assay of putative FSH preparations which fail to elicit the desired ovarian weight responses.