Natural killer (NK) activity of F344 rat spleen cells remained constant between 1 and 18 months of age under specific pathogen-free (SPF) conditions. Between 18 and 24 months of age, however, there was a dramatic decline in activity which remained at a low baseline throughout the normal lifespan. Removal of adherent cells on G-10 Sephadex columns revealed age-related changes in adherent cell regulation of NK activity. Young (4-6 week) NK activity was consistently decreased by adherent cell removal while old (24-30 month) NK activity was slightly but reproducibly increased. Moreover, splenic macrophages from old rats purified by adherence to microexudate-coated surfaces were highly suppressive to young nonadherent NK activity. A role for endogenous prostaglandin (PG) in suppressed old rat NK activity was suggested by the effectiveness of anti-PGE2 in vivo to boost old NK activity. Although old rat NK activity was boosted to a relatively greater extent by interferon (IFN) in vitro than was young NK activity, IFN-boosted NK activity of old rats was much more sensitive to PGE2 inhibition than was IFN-boosted young rat NK activity. IFN treatment in vitro or poly(I:C) treatment in vivo induced protection against PGE2 inhibition of NK activity in young rats, while no resistance to PGE2 inhibition was induced in old rat NK cells by similar treatments. In vivo, the same protocol of IFN administration which boosted young rat NK activity further suppressed old rat activity. These results support the hypothesis that immunosuppression related to aging, which supersedes the boosting effect of IFN, involves the combined effects of suppressor macrophages (via PGE2) and intrinsic changes in effector (NK) cells which render them more sensitive to PGE2 inhibition.