A method is described to enable the investigation of luteal tissue that is formed in the absence of ovarian theca lutein cells. Immature rats were given a s.c. injection of pregnant mare's serum gonadotropin (PMSG) followed 48 h later by an i.v. injection of human chorionic gonadotropin (hCG). Both ovaries were removed 8 to 10 h after the injection of hCG, the preovulatory follicles punctured, and the granulosa cells expressed into medium using gentle pressure with a dissecting scalpel. The cells were centrifuged at low speed (50 X g) for 5 min, resuspended and recentrifuged. The cells were aspirated into PE 90 polyethylene tubing, the tubing sealed at one end and then placed into a microcapillary hematocrit tube. The cells were packed by centrifugation in a microhematocrit centrifuge for 2 min, after which they were autotransplanted by injecting them beneath the kidney capsule. Progesterone, determined in blood samples obtained 4 through 12 days after transplantation, was elevated on most days compared to ovariectomized controls without cell transplants. In a second experiment, serum progesterone in ovariectomized-adrenalectomized rats with autotransplants was also elevated (4 to 6 ng/ml) compared to controls without cell transplants in which progesterone was about 0.1 ng/ml. Morphology of transplants revealed luteal cells with considerable vascularity and with the typical appearance of cells observed in in situ corpora lutea. This procedure, with modifications to include homotransplants in inbred strains, holds promise for investigation of the endocrine characteristics of granulosa lutein cells devoid of cells of thecal origin.