With the aim of further elucidating the role of the epithelial Cl- conductance and its defect in cystic fibrosis (CF) patients we studied the properties and regulation of the Cl- conductance in primary cultures of human nasal polyp epithelia. To facilitate microelectrode punctures and to gain access to the cytoplasmic compartment for injection of antibodies, we prepared giant cells using a polyethylene-glycol fusion technique. The membrane potential (Vm) and resistance (Rm) and their responses to ionic substitutions in the bath were measured under control conditions and in the presence of different secretagogues. In non-CF cells Vm averaged-12.5 mV (SD +/- 6.6 mV, n = 69) and was independent of time after fusion, while Rm dropped from 12.4 +/- 7.3 M omega (n = 51) to 3.5 +/- 5.5 M omega (n = 24) in the 2nd week after fusion. The low Vm values reflected a vanishing K+ conductance in the presence of a dominating Cl- conductance that increased with time. In young cells, a Cl- conductance prevailed which could be stimulated by application of the Ca2+ ionophore, A23187, or of carbachol. As determined in CF cells, it had an outwardly rectifying current/voltage (ilV) relationship and exhibited the selectivity sequence I- > Br- > Cl- > F- > isethionate (ISE-) both in Vm and Rm measurements. With increasing age after fusion, a Cl- conductance prevailed in non-CF cells which could be stimulated by cyclic adenosine monophosphate (cAMP) or forskolin and which was downregulated by A23187. It had a linear ilV relationship and exhibited the selectivity sequence Br- > Cl- > I- > F- > ISE- if determined from Vm measurements, but a sequence of Cl- > Br- > F- = ISE- > I- if determined from Rm measurements. This points to multiple-ion pore behaviour of the respective Cl- channel. In agreement with observations described in the following publication, the results suggest that the cAMP-regulated Cl- conductance corresponds to the CF-gene product while the molecular nature of the Ca(2+)-regulated Cl conductance is not yet known.