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The effects of short-term simulated microgravity on the lymph dynamics of rat lymph nodes were investigated using a combination of Bollman's cage and head-down tilt (HDT). Efferent lymphatics of the iliac and mesenteric lymph nodes were cannulated for the collection of lymph. There was no significant difference in lymph flow rate from the iliac lymph nodes between non-HDT (control) and HDT rats. Lymph flow rate from the mesenteric lymph nodes in HDT rats was slightly higher than that obtained with the control. The cell count obtained from the iliac lymph nodes in HDT rats was significantly larger than those of the controls, while no significant difference in the number of cells from the mesenteric lymph nodes was observed between the control and HDT groups. The cells from the iliac lymph nodes in the control and HDT rats were mostly lymphocytes. The distribution of subsets of lymphocytes (CD3+, CD4+, CD8a+, and CD45R+) from the iliac lymph nodes in HDT rats was not significantly different from the subsets of lymphocytes in the control. Immunization did not affect the distribution of lymphocyte subsets from the iliac lymph nodes in the control and HDT groups. There was no significant difference in the concentrations of lymph albumin in iliac afferent or efferent lymphatics between the control and HDT groups. These findings suggest that HDT posture in Bollman's cage induces transient output of lymphocytes from the iliac lymph nodes of rats in vivo without changing the flow rate, lymphocyte subsets, or concentration of albumin.