Hydrostatic stress-time data obtained in tests with thermoplastic pipe are analyzed and interpreted. A method of extrapolation to periods greater than the actual test period is developed. A test method to develop hydrostatic design stresses, based on this data, is discussed. The plastics considered are acrylonitrile-butadicne-styrene (ABS), polyethylene (PE), poly vinyl chloride (PVC), and cellulose acetate butyrate (CAB). Data obtained at higher temperatures are discussed briefly. The above information and consideration serves as the basis for the hydrostatic design stresses now incorporated in the new standard dimension ratio-pressure rated ASTM thermoplastic pipe specifications.