ABSTRACTDrill‐stem tests may be useful for such hydrogeological purposes as regional ground‐water or geothermal resource studies. Water samples, hydraulic head and hydraulic conductivity may be obtained from these tests.Analysis of 41 drill‐stem tests conducted for a regional ground‐water study reveals that maximum pressures recorded during a test should exceed 30 percent of the full scale of the pressure gauge used in the case of bourdon‐type instruments. Use of drill‐stem‐test data without consideration for percent deflection of the pressure recorder, even with evaluation of mechanical quality will mean that individual values of pressure head may be as much as 30 percent different from that which would be obtained in an observation well. The error is random and has a median value approximating that which would be observed in a well completed over the same interval. Use of large numbers of drill‐stem tests will produce hydraulic head maps which agree with those produced by conventional ob