The mechanism underlying the inhibition of Na reabsorption from exocrine secretions in cystic fibrosis (CF) is not known. In the animal CF phenocopy of Martinez, chronic reserpine (RS) administration to rats causes histological and physiological changes in exocrine glands similar to those in CF. To examine the effect of chemical sympathectomy on Na transport, we studied the onset of inhibition of Na reaborption from parotid saliva in male Sprague-Dawley (SD) rats given i.p. RS (0.5 mg/kg) daily. Saliva stimulated by i.p. pilocarpine was collected by cannulating the parotid duct. A single RS dose caused a marked increase in Na content within 24 hr in rats from our colony (OU). Na content increased linearly with the duration of RS administration. Saliva of SD rats from Sasco Inc. (SI) contained much less Na than that of OU rats at all saliva flow rates below 70 ul/min/g gland wet weight. RS administered to SI rats caused no demonstrable increase in saliva Na content until the 4th day. Na increased only to levels seen in untreated OU rats. These results indicate that both the intrinsic activity and the susceptibility to inhibition of the ductal Na transport system of exocrine glands show specific differences. Such differences complicate the use of uncharacterized rat strains for the bioassay of CF factors but may provide valuable insights into the biological regulation of Na transport in the exocrine gland duct.