The effects of nicotine on intestinal and tracheal mucosa and epithelial cells were studied in vivo and in vitro. Rats received 108 microM nicotine in their drinking water for 10 days. Intestine and trachea were removed and prepared for X-ray microanalysis, transmission electron microscopy and immunocytochemistry. X-ray microanalysis of freeze-dried cryosections of intestine and trachea showed an increase in Na and Cl, and a decrease in K in the lamina propria of the intestine and the epithelial cells and the submucosal compartment of the trachea. Analysis of frozen-hydrated trachea, in order to determine ionic changes in the airway-surface liquid (ASL), indicated that the Na and Cl content in the ASL decreased after nicotine treatment. Immunocytochemistry showed upregulation of ICAM-1 in the submucosal connective tissue of the trachea and the lamina propria of the intestine. Transmission electron microscopy showed a somewhat increased number of eosinophils in the lamina propria of nicotine-treated rats, increased edema in the submucosal connective tissue, and a somewhat increased number of damaged basal cells in the trachea of nicotine-treated rats, compared to the controls. These data indicate that nicotine may evoke an inflammatory reaction, in particular in the trachea, that could cause cell damage and with that changes in the ionic relations of the epithelial cells. The in vitro experiments showed that nicotine could directly affect ion transport by inhibiting cAMP-stimulated (but not ATP-stimulated) chloride efflux from cultured respiratory epithelial cells. This indicates that apart from indirect effects via inflammation, nicotine can directly affect the ionic homeostasis of the cells and the composition of the airway-surface liquid.