The purpose of these studies was to determine whether treatment of newborn rats with exogenous FSH or hCG would alter the concentration or size of testicular macrophages. Animals were injected once daily with various doses of FSH, hCG, or vehicle for 8-10 days beginning the day following birth. After immunohistochemical labeling of the macrophages with a monoclonal antibody specific for rat macrophages, the concentration and size of macrophages were determined by use of a point-counting method. Body weight, testis weight, and serum levels of testosterone and FSH were also measured. It was found that hCG significantly increased the concentration of macrophages within the interstitium but did not affect the size of the cells. Both testicular weight and serum testosterone concentrations increased after hCG treatment. Although FSH increased the weight of the testis, neither the size nor concentration of macrophages was altered. These results raise the possibility that the number of macrophages within the interstitial compartment of the normally maturing rat testis is under the control of LH.