Caffeine in 10-2 M concentration per se activates ryanodine receptors (RyR) in vitro, thereby increasing the intracellular concentration of Ca2+ ([Ca2+]i). In general opinion, caffeine applied in vivo in much lower doses does not affect [Ca2+]i in neurones. However, it was recently demonstrated that caffeine in low concentrations in vitro potentiates evoked Ca2+ release in neurones via RyR. Microdialysis of the rat dentate gyrus (DG), combined with measurement of 45Ca2+ efflux, has been used in our laboratory to study in vivo NMDA-evoked calcium induced calcium release (CICR) via RyR. The aim of the present microdialysis study was to investigate in vivo effects of caffeine, applied systemically in a pharmacologically-relevant dose, and locally in the dialysis medium in very high concentration, on the NMDA-evoked CICR in DG neurones. To ensure steady brain concentration of caffeine, its systemic (i.p.) administration in a dose of 40 mg/kg was followed by a continuous i.p. infusion of 80 mg/kg/min and application of 0.4 mM caffeine in the dialysis medium. The results demonstrated that in the rat DG, local administration of 50 mM caffeine significantly stimulates a spontaneous 45Ca2+ efflux and its release induced by 5 mM NMDA. However, systemic administration of caffeine had no effect on spontaneous and NMDA-induced 45Ca2+ release in the rat DG, which supports the view that caffeine, applied in vivo, even in high doses, does not influence CICR in brain neurones.