The effects of microgravity on the biophysical properties of frog labyrinthine hair cells have been examined by analyzing calcium and potassium currents in isolated cells by the patch-clamp technique. The entire, anesthetized frog was exposed to vector-free gravity in a random positioning machine (RPM) and the functional modification induced on single hair cells, dissected from the crista ampullaris, were subsequently studied in vitro. The major targets of microgravity exposure were the calcium/potassium current system and the IA kinetic mechanism. The amplitude of ICa was significantly reduced in microgravity-conditioned cells. The delayed current, IKD (a complex of IKV and IKCa), was drastically reduced, mostly in its IKCa component. Microgravity also affected IKD kinetics by shifting the steadystate inactivation curve toward negative potentials and increasing the sensitivity of inactivation removal to voltage. As concerns the fast, transient potassium current, IA, the I-V and steady-state inactivation curves were indistinguishable under normogravity or microgravity conditions; conversely, IA decay systematically displayed a twoexponential time course and longer time constants in microgravity, thus potentially providing a larger K charge; furthermore, IA inactivation removal at 70 mV was slowed down. Stimulation in the RPM machine under normogravity conditions resulted in minor effectson IKD and, occasionally, incomplete IA inactivation at 40 mV.Reduced calcium influx and increased K repolarizing charge, to variable extents depending on the history of membrane potential, constitute a likely cause for the failure in the afferent mEPSP discharge at the cytoneural junction observed in the intact labyrinth after microgravity conditioning.