Cat splenic slices prelabelled with [3H]-noradrenaline were incubated in oxygenated Krebs-bicarbonate solution at 37 degrees C, and the spontaneous total 3H release into different incubation media monitored. In normal Krebs bicarbonate solution, the spontaneous tritium fractional release amounted to 3.7% of the tissue radioactivity content per 5 min collection period. Tetraethylammonium (TEA) increased spontaneous transmitter release in a concentration-dependent manner; the release was maximal at 30 mM and was 3.5 times the basal release. 4-Aminopyridine (4-AP) also enhanced the spontaneous release of tritium. The response increased linearly with 4-AP concentration (1-10 mM). With 10 mM 4-AP, the release was as much as 6 times the basal transmitter release. Guanidine was much less potent than either TEA or 4-AP. The secretory response to TEA or 4-AP was little affected by changes in external Ca2+, Mg2+, Na+, Cl-, H2PO4- or by tetrodotoxin. However, transmitter release evoked by TEA or 4-AP strongly depended upon the concentration of HCO3- of the incubation solution; in fact, the secretory response varied almost linearly between 1 and 25 mM HCO3-. The mechanisms underlying these effects are probably related to the well-known ability of TEA and 4-AP to block K+ conductance that would cause depolarization of the splenic sympathetic nerve terminals. The HCO3- requirements for the secretory response are probably related to the ability of CO2/HCO3- solutions to mobilize and release Ca2+ from intracellular organelles.