Spontaneous pituitary tumors occurring in groups of 100 Sprague-Dawley (SD) and 100 Fischer 344 (F344) rats of each sex on a 2-year aging study were characterized by immunocytochemistry. The SD strain had a total of 75 tumors with 10% in males and 65% in females. Tumors immunoreactive for prolactin (PRL) alone were the most common tumor (64%) with the immunonegative tumor being the second most common (17.3%). F344 rats had a total of 62 tumors with 26% in males and 36% in females. The majority of the tumors were reactive for prolactin alone (56.5%) and tumors reactive for both growth hormone (GH) and PRL were the second most common (21%). Most tumors were immunoreactive for only 1 hormone; however, both strains had tumors that expressed multiple hormones in unusual combinations.