The suppressive effect of hypercapnia on triggered epileptiform depolarisations (ED) has recently been attributed to altered adenosine levels, while a contribution of lowered intracellular pH (pHi) was denied. Using rat and human neocortical slices as well as organotypic cultures (OC), we tested whether a drop of pHi can suppress 0-Mg-induced EDs. Layer 3 pyramidal neurons were intracellularly recorded in bicarbonate-buffered artificial cerebrospinal fluid. pHi was measured at the surface of BCECF- AM loaded slices, and in neurons or soma-near dendrites loaded with pyranine (HTPS). Hypercapnia (PCO2: 80–100 mmHg) lowered pHi by up to 0.3 pH units under all conditions and suppressed ED frequency in acute slices of human and rat neocortex. Surprisingly it largely failed to inhibit EDs in OC. Propionate (10 mM, for 10 min) reduced pHi at the surface and in superficial neurons by up to 0.2 pHi units. However, it failed to evoke a neuronal or dendritic acidification at a depth of >40mm. Effects of propionate on ED seen in acute slices were similar to those evoked by hypercapnia, although they were less pronounced. In contrast, propionate had no suppressive effect on ED in OC. We suggest that propionate effects are limited by diffusion and that a lowering of pHi preferentially affects network or pacemaker structures hardly conserved in OC.