The anterior pituitary gland is a dynamic and complex endocrine gland that synthesizes and secretes trophic hormones from distinct endocrine cells including mammotrophs (prolactin cells or lactotrophs) and somatotrophs (growth hormone cells) to play essential roles in the maintenance of homeostasis, metabolism, reproduction, growth, and lactation. Recently, the existence of a novel anterior pituitary cell type, the mammosomatotroph which contained both growth hormone (GH) and prolactin (PRL) was proposed to function as a transitional cell or progenitor cell between somatotrophs and mammotrophs under various physiological conditions. The present study investigated the spatial distribution patterns of somatotrophs, mammotrophs, and mammosomatotrophs by counting the number of immunopositive cells at three positions in each of 5 regions at 3 levels in the porcine anterior pituitary from newborn and prepubertal stages of pigs (day 1, day 45, and day 90) by using fluorescence immunocytochemistry. There were no significant differences among the total somatotrophs per counting area across the three age groups. However, significant increases were observed among the total numbers of mammotrophs and mammosomatotrophs across the three age groups (day 1: day 45 and day 1: day 90, P<0.01). There were distinct spatial changes in these cell types across different regions, positions, and levels in the anterior pituitary. Somatotrophs were densely distributed in lateral wings of the anterior lobe (regions 1 and 5) whereas mammotrophs were numerous in shoulder areas (regions 2 and 4) in all age groups. In the center (region 3), near the intermediate lobe (positions a and b) at the proximal level, there was a significant decrease in the total number of somatotrophs from day 1 to day 90 (P<0.01). However, mammotrophs and mammosomatotrophs significantly increased from day 1 to day 90 (P<0.01). From proximal to distal level, in the center and the outer surface of the anterior lobe (position c), the number of somatotrophs, mammotrophs, and mammosomatotrophs significantly increased (P<0.01). The results of these studies strongly suggest regional specificity of cellular transformation or interconversion to facilitate GH and PRL secretion as the need for endocrine regulation during the rapid growth period in the young pig.