A 65-ac (26.3-ha) cedar-hemlock catchment in northern Idaho was sprayed with a 5% aqueous emulsion of silicone oil to reduce transpiration. The antitranspirant, Dow Corning XEF-4-3561, was applied by helicopter at the rate of 40 gal./ac (375 1/ha). From June 1 to September 15, 1974, streamflow, soil water, and leaf water potential were monitored on the 65-ac (26.3-ha) treated watershed and an adjacent 50-ac (20.2-ha) control watershed. By using the paired watershed method with regression equations based on 6 years of pretreatment data, predicted and observed streamflow were obtained for the period June 1 to September 15. Differences between predicted and observed streamflow were compared by using the paired ‘t’ test and found significant at the 97.5% confidence level. During a 63-day period a 12% increase in streamflow resulted from the antitranspirant application. During this same period, soil water storage was greater on the treated watershed than on the control watershed; however, the difference was not statistically significant. Seasonal leaf water potential in cedar indicated that stress was greater on the control watershed than on the treated watershed. Systematic differences in leaf water potential for hemlock did not occur.