Depressed immune function is a well-documented effect of spaceflight. Both in-flight studies and ground-based studies using microgravity analogs, such as Rotating Wall Vessel (RWV) bioreactors, have demonstrated that mitogen-stimulated T lymphocytes exhibit decreased proliferation, IL-2 secretion, and activation marker expression in true microgravity and the dynamic RWV-culture environment. This study investigates the kinetics of RWV-induced T lymphocyte inhibition by monitoring the ability of Balb/c mouse splenocytes to become activated under static culture conditions following concanavalin A (Con A) stimulation in an RWV. Splenocytes were stimulated with Con A and cultured for up to 24-hrs in the RWV before being allowed to "recover" under static culture conditions in the continued presence of Con A. The T lymphocyte fraction of splenocytes was assayed during the recovery period for IL-2 secretion, expansion of the T lymphocyte population, and expression of the activation marker CD25. Our results indicate that CD25 expression was not affected by any duration of RWV-exposure. In contrast, proliferation and IL-2 secretion were inhibited by greater than 8- and 12-hrs of exposure respectively. Culture in the RWV for 24-hrs resulted in a near-complete loss of cellular viability during the recovery period, which was not seen in cells maintained in the RWV for 16-hrs or less. Taken together, these results indicate that for up to 8-hrs of RWV culture activation is not significantly impaired upon return to static conditions; longer duration RWV culture results in a gradual loss of activation during the recovery period most likely due to decreased T cell viability and/or IL-2 production.