The effects of intracellularly injected inositol trisphosphate (IP3) and nonhydrolyzed GTP analogue (Gpp[NH]p) on intracellular concentration of calcium ions [Ca2+]in was determined by fluorescence signals obtained from Helix pomatia neurons. At the same time these neurons were studied under the current clamp conditions. The IP3 and Gpp[NH]p injection caused a long-lasting [Ca2+]in increase in all neurons. Removal of the Ca2+ ions from external solution did not change the [Ca2+]in values. It is suggested that there is IP3- and GTP-dependent release of Ca2+ from intraneuronal stores in the Helix pomatia neurons.