The prompt nature of the postcastration rise of LH and FSH in the male rat suggests that there may be changes in hormonal storage patterns in developing castration cells. To study this phenomenon, pituitaries from male rats 12 and 24 h after castration were stained with antibovine LHβ (1:10,000) and antihuman FSHβ (1:8000) and two immunocytochemical methods: the peroxidase-antiperoxidase complex technique and the avidin-biotin complex technique. The postcastration rise in serum gonadotropins was determined by RIA. Developing castration cells showed dilated profiles of rough endoplasmic reticulum which contained stain for LH or FSH, or were empty and vacuolated. They did not stain for ACTH. Three stereometric parameters were evaluated: 1) the percentage of LH or FSH gonadotropes in the cell population, 2) the volume density or concentration of LH or FSH gonadotropes in the pituitary cell mass, and 3) the average area fraction (a) of individual gonadotropes, measured with a calibrated grid in the light microscope. The a reflects changes in cell size. As early as 12 h after castration, the volume density of stained FSH gonadotropes increased. The a of LH gonadotropes increased, and the percentage of gonadotrope cells that stored both hormones rose from 70% in the intact rat to 91%. Twenty-four hours after castration, the percentage of LH and FSH gonadotropes increased from 9–10% (in the intact rat) to 14–15% of the cell population. Only the LH cell population showed an increase in area 12 and 24 h after surgery. The FSH cells, which are larger in the intact rat, increased in frequency, but not in average a. Our data are consistent with physiological studies that describe a rapid secretory response to castration and maintenance of gonadotropin storage levels in the pituitary 2 days after surgery. We also propose that the increased castration cell volume, which reflects increases in cell number and size, may provide the surface area needed to support the increased number of GnRH receptors detected 24 h postsurgery in male rats (3).