A growing body of evidence hints at intracellular free protons to be involved in the modulation of electric activity of cortical neurones. In this study we demonstrate that application of the weak acid propionate (2.5-20 mM) transiently lowers intracellular pH (pH(i)) of BCECF-AM loaded CA3-neurones in hippocampal slices. The predictability of this acidification prompted us to use propionate as a tool to investigate effects of pH(i) on spontaneous bioelectric activity (SBA) and epileptiform activity (EA, induced by bicuculline, caffeine or low magnesium) of CA3 neurones: SBA and EA were transiently suppressed by 2-20 mM propionate - coinciding with the transient neuronal acidification. As activation of Na(+)/H(+)-exchangers (NHE) is involved in the recovery from neuronal acidosis and NHE-inhibition alone is known to increase the activity of intracellular free protons of hippocampal neurones, we tested the effect of the NHE-blockers amiloride (0.5-1 mM) or HOE642 (200 microM) on SBA and EA of CA3-neurones. Long-term application of NHE-inhibitors alone continuously suppressed SBA and EA, which recovered during additional exposure to the weak base trimethylamine (5-10 mM). Simultaneous administration of propionate and NHE-blockers intensified the inhibition of neuronal activity. Together, these results indicate that intracellular acidification inhibits bioelectric activity of hippocampal CA3-neurones. This supports the hypothesis that pH(i) contributes to the control of cortical excitability.