1. The sympathetic nerve terminals of the mouse vas deferens were loaded with the calcium indicator Oregon Green 488 BAPTA-1 by orthograde transport along the postganglionic nerves. Changes in the calcium concentration in the varicosity (delta [Ca2+]v) were determined following single impulses, and short (5-impulse) and long (200-impulse) trains at 5 Hz. 2. All varicosities showed a significant delta [Ca2+]v in response to every single impulse. The elevated delta [Ca2+]v declined in two phases with similar kinetics for all varicosities: a fast phase (time constant, 0.42 +/- 0.05 s) and a moderate phase (3.6 +/- 0.4 s). 3. Line scanning confocal microscopy revealed that the delta [Ca2+] of a single terminal following single impulses was smaller for the intervaricose regions than for the varicosities. 4. Blockade of the voltage-sensitive calcium channels with Cd2+ (in calcium-free solution) completely blocked the delta [Ca2+]v on stimulation. The addition of either nifedipine (10 microM), omega-conotoxin GVIA (100 nM) or omega-agatoxin TK (100 nM) showed that 47 +/- 6% of the evoked response was mediated by N-type calcium channels. 5. Ryanodine (10 microM) did not significantly change the amplitude of delta [Ca2+]v in response to short trains. 6. Spontaneous increases in delta [Ca2+]v were observed in individual varicosities, with coupling in the increase of delta [Ca2+]v between varicosities. 7. The presynaptic alpha 2-receptor antagonist yohimbine (10 microM) increased the amplitude of delta [Ca2+]v in response to five impulses (5 Hz) by 54 +/- 14%, while the alpha 2-receptor agonist clonidine (1 microM) decreased the delta [Ca2+]v by 55 +/- 4%. 8. These results are discussed in terms of the hypotheses that the increased probability for secretion at sympathetic nerve terminals which accompanies facilitation and augmentation is due to the residual delta [Ca2+]v remaining after the calcium influx following impulses and that noradrenaline acts presynaptically to decrease the probability of secretion by modifying calcium influx.