The major purpose of this study was to quantify hypergravity-induced changes in erythrocyte and thrombocyte characteristics, spontaneous and mitogen-induced lymphoblastogenesis, and capacity of splenocytes to secrete immunoregulatory cytokines. C57BL/6 mice were subjected to chronic 1, 2, and 3 G; subsets were euthanized after 1, 4, 7, 10, and 21 days of centrifugation. Erythrocyte counts, hematocrit, and hemoglobin were significantly reduced by day 21 in both centrifuged groups. Hemoglobin concentration and volume per red blood cell were generally low, but an early, transient spike above normal was noted in thrombocyte counts in the 3 G group. Fluctuations above and below normal in blood and spleen cell spontaneous blastogenesis were dependent upon the length of centrifugation time and not on the level of gravity. Depression in splenocyte responses to phytohemagglutinin and lipopolysaccharide due to gravity were noted when the data were expressed as stimulation indices. Cytokine production by spleen cells was primarily affected during the first week of centrifugation; interleukin-2 (IL-2), IL-4, and tumor necrosis factor- increased, whereas interferon- decreased. These findings, although not identical to those reported for space flight, indicate that altered gravity can influence both hematological and functional variables that may translate into serious health consequences during extended missions.