Natural killer cell cytotoxicity (NKCC) was reported to manifest a circadian rhythm, peaking during wakefulness in both human blood and rat spleen. Using F344 rats, we investigated whether such fluctuations (i) reflect changes in NK cell numbers or in cytotoxicity per cell; (ii) coincide in the blood and spleen; (iii) correspond with clearance of NK-sensitive tumor cells from the lungs and (iv) influence formation of lung metastases. Two rat groups were housed in opposite 12:12 hr lighting regimens. Two hours after the onset of light or dark, both groups were either sacrificed or intravenously inoculated with tumor cells to study the following indices: NKCC and NK cell numbers in the spleen (n โซุโฌ 29) and blood (n โซุโฌ 79), lung clearance of tumor cells (n โซุโฌ 142) and lung metastasis (n โซุโฌ 69). The tumor employed, MADB106, is an NK-sensitive mammary adenocarcinoma that metastasizes only to the lungs. The results indicated that, during the dark phase, splenic NKCC increased (37% higher lytic unit [LU] 50 ) mostly due to a 28.9% higher percentage of NK cells in the spleen. In contrast, blood NKCC decreased by 42.5% (LU 20 ) and this decline was independent of circulating NK cell numbers, which remained constant. Lung tumor clearance increased in the dark (up to 42% lower retention 9 hr after inoculation), but no corresponding changes in the number of metastases were observed 3 weeks later. We conclude that diurnal changes in rats' NKCC are organ-specific, involve changes in both cell distribution and activity and may affect short-term in vivo indices of NK tumoricidal activity.