Several previous studies have reported that a substance in exocrine products from cystic fibrosis (CF) patients is inhibitory to the transport of NaCl in several biological preparations. We have recently developed techniques for studying a target CF tissue, the eccrine sweat duct, and determined that salt absorption in the CF sweat duct appears to be decreased due to an impermeability to CI. To determine whether this property might be induced, we have examined the sweat from patients with CF for potential influences on the ion-reabsorptive and ion-selective properties of sweat ducts from normal subjects. Isolated segments of sweat ducts from healthy volunteers were microperfusedin vitrowith concentration-adjusted sweat collected after thermal stimulation from CF patients and from normal subjects. The eccrine sweat duct may be best considered as a tight epithelium through which the mucosal uptake of NaCl proceeds via separate Na+and Cl-pathways. As such, inhibitory actions of CF fluids should be exerted via an inhibition on one, or both, pathways, and should be expected to 1) inhibit the electrolyte transport activity and 2) alter the ion-selective properties of normal ductal tissue. We were unable to detect any effect of CF sweat on either property of the normal duct.