The single-well chemical tracer (SWCT) method was used to measure resident water saturation in two wells in the Ivishak reservoir of the Prudhoe Bay field. The tests, performed in 1981, were in API Wells 406 and 488, both of which had been oil-base cored through the Ivishak. The results of the tracer test in Well API 406 were interpreted by conventional SWCT simulation procedures. A three-layer simulator model was used to match the tracer production profiles measured during the test. The average water saturation obtained from the simulator match was 16 +/- 3% PV, in good agreement with the Dean-Stark analysis of the oil-base cores. The tracer-test results from Well API 488 were more difficult to interpret because of the very low water saturation at this location in the Ivishak. The SWCT method requires a hydrolysis reaction to take place in the formation to produce a product tracer. In this case, the amount of water present was so small that very little product tracer was formed. The interpreted S-w for this well was 2 +/- 2% PV, again in reasonable agree ment with the oil-base-core measured value of 3.8%.