The Soil Conservation Experiment Station located near Clarinda, Iowa, was established in 1931 to study erosion problems on Marshall silt loam. One of the original experiment started in 1932 contained plots comparing slope lengths of 36.3, 72.6 and 145.2 feet. These plots were 6 feet wide with an average land slope of 9 percent. The plots were planted to continuous corn. Originally provision was made only for measurement of total runoff and erosion. However, during the years 1938 to 1942, devices were installed for measurement of rates of runoff. Using rainfall intensity rates and runoff rate measurements, infiltration rates were determined by the graphical method of Sharp and Holtan for 10 of the largest storms selected from a total of 82 storms, with appreciable runoff, that occurred during the 5-year period. The ten storms were selected on the basis of long duration, high intensity and different times of the year. On the basis of this study the following conclusions were drawn. a) In general, mass infiltration increased with increased slope length; b) The average initial infiltration rate for all storms and all slope lengths was 0.79 inch per hour and the final infiltration rate was 0.21 inch per hour. The final infiltration rate varied with storms and slope lengths from 0.60 to 0.10 inch per hour. c) The average infiltration rate curve calculated from the mean of all curves for all storms and all slope lengths was expressed by the equation f = 0.211 + 1.019 e-0.065t This equation is in the same form as that of Horton. d) The infiltration rate was influenced slightly by increasing the slope length. The mean final infiltration rate was slightly greater from the 145-2 feet length slope than from 36.3 feet, slope length. e) Although the data are not conclusive, an interesting relationship appears to exist between the final infiltration rate and the season of the year. Final infiltration rate appears to increase up to June and July and then decrease rather sharply during August and September. The results are in agreement with those found by Borst et al. f) An attempt was made to relate Antecedent Precipitation Index to infiltration rates. Antecedent Precipitation Indices are used as expression of the wetness of the soil. The relation between API and mass infiltration as percent of the total rainfall appeared to be the best of several tested. Curves have been .prepared presenting the relationship between API and infiltration rate at various times after rainfall started. Infiltration rate is greater with decreasing API's at the Btart of a storm. After 60 minutes of excess rainfall, however, the infiltration rate curves for all API's converge.