Loading of mouse motor nerve terminals with EGTA-AM, but not with BAPTA-AM, inhibited the release of the neurotransmitter in response to stimulation of the nerve with rare (0.3 Hz) "single" pulses. During rhythmic stimulation with short (50 EPP) high-frequency (20 Hz) series, BAPTA-AM buffer modified burst pattern in a dose-dependent manner: it replaced the phase of initial facilitation by persistent depression of secretion and decreased its plateau level at the end of the burst. In contrast, loading of the nerve terminals with EGTA-AM buffer produced no effect on the phase of initial facilitation, but decreased the plateau level to the same degree as BAPTA-AM did. Probably, the different effects of both buffers on secretion of neurotransmitter reflect peculiarities of involvement of fast and slow Ca(2+)signals of motor terminals in single and rhythmic release of the neurotransmitter.