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A measurement-while-drilling (MWD) neutron porosity tool has been designed that is superior to wireline porosity tools in several respects. Longer sample periods and greater porosity sensitivity reduce the statistical uncertainty in the MWD porosity measurement. Most of the borehole effects that perturb the measurement (salinity, mud weight, and borehole size) are reduced or nonexistent because the drill stem virtually fills the hole during drilling. Formation effects, lithology, and salinity changes, are comparable to or less than those for the wireline measurement. A sophisticated computer model was used to develop the initial design for this instrument and to estimate its response characteristics. Simulated response data are compared with experimental data to demonstrate the validity of the model. The response characteristics are superior to those of the wireline device.