Examination of pituitaries from young and adult turtles representing four families, reveals that in addition to the abundant juxtaneural pars tuberalis (juxPT) found in this class of reptiles, there is generally a substantial amount of pars tuberalis (PT) tissue closely associated with the pars distalis (PD). The PT forms a cortical layer especially conspicuous around the anterior tip of the PD in some species (Trionyx, Kinosternon, Sternotherus), or it forms a thick dorsal layer of tissue irregularly extending onto the sides of the PD in others (Pseudemys, Chrysemys, Lepidochelys, Chelonia). Immunocytochemical studies using unlabelled second antibody and peroxidase-antiperoxidase reveal that in turtles of all ages, the PT tissue allied with the PD (the PTinterna) is composed primarily of cells containing glycoprotein hormones (FSH, LH and TSH), especially the gonadotropins. The juxPT, however, consists mainly of secretory cells unstained by the antisera tested and includes only a small number of gonadotropes and thyrotropes. Although usually widely distributed in the testudinate adenohypophysis, the great majority of gonadotropes and thyrotropes present in the hatchling are in the PTinterna. It is probable that a concentration of these cells in the PTinterna is widespread among vertebrates. In all turtles examined, lactotropes occur principally in the anterior and ventral part of the PD proper; somatotropes are posterior and dorsal. Corticotropes are concentrated as the lactotropes in the anterior PD, but some are also scattered throughout the posterior half of the gland. Lactotropes, corticotropes, and with a few exceptions, somatotropes, do not occur in PT tissue in the turtle.