1. Standard intracellular recording techniques with ‘sharp’ micropipettes were used to evoke action potentials (APs) in acutely dissociated adult nodose neurones. 2. APs induced a transient increase in [Ca2+]i (a calcium transient), recorded with fura-2, that was dependent upon [Ca2+]o and the number of APs. Over the range of one to sixty-five APs, the relation between the amplitude of the calcium transient and the number of APs was well fitted by a rectangular hyperbola (chi 2 = 3.53, r = 0.968). From one to four APs, the calcium transient-AP relation can be described by a line with a slope of 9.6 nM AP-1 (r = 0.999). 3. Charge movement corresponding to Ca2+ influx evoked by a single AP was 39 +/- 2.8 pC (mean +/- S.E.M.) and did not change significantly during trains of one to thirty-one APs (P < 0.05). 4. Caffeine (10 mM), a known agonist of the ryanodine receptor, produced an increase in [Ca2+]i. The caffeine-induced rise in [Ca2+]i was attenuated (by > 90%) by lowering [Ca2+]o, and by ryanodine (10 microM), 2,5-di(t-butyl)hydroquinone (DBHQ, 10 microM), or thapsigargin (100 nM). 5. Neurones incubated with ryanodine, DBHQ or thapsigargin required at least eight APs to evoke a detectable calcium transient. These reagents did not significantly affect Ca2+ influx (P < 0.05). In the presence of these inhibitors, the calcium transient-AP relation exhibited slopes of 1.2, 1.1 and 1.9 nM AP-1 for ryanodine, DBHQ and thapsigargin, respectively. When compared with the slope of 9.6 nM AP-1 in non-treated neurones, it appears that Ca2+ influx produced by a single AP is amplified by ca 5- to 10-fold.