The routes of Ca2+ entry in response to N-methyl-D-aspartate (NMDA) and K+ depolarisation in cerebellar granule cells have been investigated using fura-2 fluorescence to measure intracellular Ca2+ concentrations ([Ca2+]i) and Mn2+ quench of fura-2 fluorescence as an index of Ca2+ entry. Removal of extracellular Na+ did not affect the [Ca2+]i elevation or the rate of Mn2+ quench of fura-2 fluorescence in response to NMDA (100 microM). K+ (25 mM) produced a [Ca2+]i increase which showed a 27% reduction in the presence of the NMDA channel blocker MK-801 (10 microM), whereas no reduction was detected in 50 mM K+ stimulated [Ca2+]i increases. K+ (25 and 50 mM)-stimulated Mn2+ quench rates were not significantly reduced by MK-801. These results demonstrate that NMDA primarily stimulates Ca2+ entry directly through the NMDA receptor without a major component of Ca2+ entry through voltage-gated Ca2+ channels (VGCCs). Under conditions which minimise the accumulation of endogenous glutamate, K+ depolarisation elicits a Ca2+ influx resulting mainly from activation of VGCCs. Additionally, these results show Mn2+ quench of fura-2 fluorescence to be a sensitive and definitive assay of Ca2+ entry through the NMDA receptor and VGCCs.