With the use of monosodium L-glutamate (MSG) treatment and ovariectomy, we studied how the arcuate nucleus and ovary are involved in the maturation of GH, PRL, and nongranulated cells in the anterior pituitary lobe during postnatal development. From day 1 to day 7 female pups were injected sc daily with MSG (2 mg/g BW) (MSG mice) or distilled water (control mice). Just before puberty (at 30 days of age) some control and MSG mice were ovariectomized (OVX) (control + OVX mice and MSG + OVX mice, respectively). All animals were killed at 60-70 days of age by decapitation. Populations of GH, PRL, and nongranulated cells in the anterior pituitary lobe were calculated by applying morphometric procedures. Percentages of GH, PRL, and nongranulated cells in MSG mice were not different from those of control mice. In control + OVX mice the percentage of GH cells was greater than that of control mice, while PRL cells were markedly decreased. On the other hand, in MSG + OVX mice the percentage of PRL cells was less than that of MSG mice, while GH cells showed no difference; nongranulated cells were increased. The number of parenchymal cells in the anterior pituitary lobe in MSG mice was approximately two thirds that of control mice. The parenchymal cells in control + OVX mice were the same in number as those in control mice, while those in MSG + OVX mice were approximately half the number of the parenchymal cells in MSG mice. Numbers of GH and PRL cells in MSG mice were half those of control mice. There was no difference in the number of nongranulated cells between MSG and control mice. GH cells in control + OVX mice were more abundant than those of control mice, while PRL cells were fewer. On the other hand, both GH and PRL cells in MSG + OVX mice were fewer when compared with those of MSG mice. The numbers of nongranulated cells in most groups showed no significant differences among each other. All of these data taken from control mice were the same as those taken from normal mice. These results suggest that the arcuate nucleus in the hypothalamus stimulates the differentiation of GH and PRL cells during postnatal life, and that the ovary stimulates the differentiation of PRL cells through a direct effect on the anterior pituitary lobe but inhibits that of GH cells via the arcuate nucleus in the hypothalamus, while the differentiation of nongranulated cells is little affected.