The ultrastructural changes occurring in the adenohypophysis (AH) of the anadromous sea lamprey, Petromyzon marinus, during metamorphosis (stages one through seven) were examined. The rostral pars distalis initially contains one granulated (secretory) cell type A and one nongranulated type I cell. A second granulated cell (type B) appears during the later stages (stages six and seven) of metamorphosis. The most pronounced ultrastructural changes take place in the caudal pars distalis (CPD). Initially, most cells (80-90%) are nongranulated cells type II and some type I. Granulated type C and D cells form the remainder of the CPD. Almost all cells during stages three and four demonstrate a marked increase in synthetic activity evident by conspicuous Golgi regions, abundant rough endoplasmic reticulum (RER), and increased cell volume. Most cells are sparsely granulated. Secretory cell types C and D and, two new cell types, E and F, are present. Synthetic activity subsides by stage five. Most cells (80-90%) during stages five through seven are granulated. Type E are most prevalent with variable numbers of types C and D and few type F. Nongranulated cells now represent only 10-20% of the CPD. The increase in granulated cells occurs at the expense of type II cells that differentiate into granulated cell types. The fine structure of the pars intermedia throughout metamorphosis remains similar to that of the larva. Most cells are granulated, highly vesiculated type G cells. A few nongranulated type I cells are also present. The functional significance of the secretory cells in the AH is related to the requirement for an intact pituitary gland for the initiation and completion of metamorphosis.