We investigated the relationship between the absorptive pathway and the immune responses of the lung, particularly the phagocytic function of alveolar macrophages (Mphi) after oral administration of D-limonene in rats. D-Limonene was orally administered in oily solution with a stomach tube in thoracic duct-cannulated rats, and the lymphatic output of D-limonene was measured. D-Limonene levels reached a maximum in thoracic duct lymph and lung 3 h after its oral administration. It also significantly increased in bronchoalveolar lavage fluid (BALF) and alveolar Mphi, in which there was frequently a focal exudation of lipid droplets containing D-limonene into the alveolar cavity through alveolar capillary walls. Second, D-limonene orally given to rats (250, 500, 1,000 mg/kg/d) for 8 consecutive days resulted in a marked increase in both the number and the phagocytic activity of alveolar Mphi compared to the controls. BALF from rats dosed with D-limonene (1,000 mg/kg/d) enhanced the phagocytic activity of alveolar Mphi from control rats because the dose was prolonged. The activity of alveolar Mphi following in vitro incubation with D-limonene also increased in a dose-dependent manner. An oral administration of D-limonene enhanced the Con A-stimulated proliferation of splenocytes. These results suggest that D-limonene taken up from the thoracic duct lymph moves to the lung and directly activates the immune response of alveolar MO there, or indirectly activates it through activated lymphocytes.