The magnitude and frequency of floods on natural-flow streams in Massachusetts, with drainage areas between 0.25 square mile and 497 square miles, may be estimated from drainage area, main channel slope, a precipitation index, and a storage factor. Multiple-regression techniques were used to define the relationship between basin and climatic characteristics, and flood peaks for 113 gaging stations. Flood-frequency and magnitude data at the gaging stations were computed using the guidelines recommended by the U.S. Water Resources Council. It was found that flood peaks of rural, unregulated Massachusetts streams can best be defined by dividing the State into an EASTERN MASSACHUSETTS and a WESTERN MASSACHUSETTS flood-frequency region. The regression equations relate peak discharges with 0.5, 0.2, 0.1, 0.04, 0.02, 0.01, 0.005, and 0.002 exceedance probabilities to basin and climatic parameters. Standard errors of estimate ranged from 42 to 70 percent for the eastern river basins and from 32 to 68 percent for the western river basins. The defined estimating relations do not apply to streams where the flood flows are significantly affected by regulation where the usable man-made storage is over 4.5 million cubic feet per square mile, or by diversions or urbanization, or where the basin indices are outside a specified range. Also, these equations should not be used for basins in eastern Plymouth County, or Barnstable, Dukes, or Nantucket Counties where the available data are insufficient to determine the influence of high infiltration and storage capacities of drainage basins on flood flows. (Woodard-USGS)