Neuroepithelial cells (NECs) of the fish gill are respiratory chemoreceptors that detect changes in O2 and CO2/H(+) and are homologous to type I cells of the mammalian carotid body. In zebrafish (Danio rerio), stimulation of NECs by hypoxia or hypercapnia initiates inhibition of K(+) channels and subsequent membrane depolarisation. The goal of the present study was to further elucidate, in zebrafish NECs, the signalling pathways that underlie CO2/H(+) sensing and generate intracellular Ca(2+) ([Ca(2+)]i) signals. Breathing frequency was elevated maximally in fish exposed to 5 % CO2 (~37.5 mmHg). Measurement of [Ca(2+)]i in isolated NECs using Fura-2 imaging indicated that [Ca(2+)]i increased in response to acidic hypercapnia (5 % CO2, pH 6.6) and isocapnic acidosis (normocapnia, pH 6.6), but not to isohydric hypercapnia (5 % CO2, pH 7.6). Measurement of intracellular pH (pHi) using BCECF demonstrated a rapid decrease in pHi in response to acidic and isohydric hypercapnia, while isocapnic acidosis produced a smaller change in pHi. Intracellular acidification was reduced by the carbonic anhydrase inhibitor, acetazolamide, without affecting [Ca(2+)]i responses. Moreover, intracellular acidification using acetate (at constant extracellular pH) was without effect on [Ca(2+)]i. The acid-induced increase in [Ca(2+)]i persisted in the absence of extracellular Ca(2+) and was unaffected by Ca(2+) channel blockers (Cd(2+), Ni(2+) or nifedipine). The results of this study demonstrate that, unlike type I cells, extracellular H(+) is critical to the hypercapnia-induced increase in [Ca(2+)]i in NECs. The increase in [Ca(2+)]i occurs independently of pHi and appears to originate primarily from Ca(2+) derived from intracellular stores.