A modulated-source Eo¨tvo¨s experiment was performed at the Little Goose Lock on the Snake River during the summers of 1988 and 1990. Although results of the first experiment were published by the author in 1989, the results from a more sophisticated version of the experiment performed in 1990 are published here for the first time. The 1990 experiment involved a freely oscillating toroidal Cu-Pb pendulum suspended from a 5-&mgr;m tungsten fiber having a period of 790 sec and a torsion constant of 0.00035 erg/rad. The lock contained≈1.7×108 kgof water which could be filled or drained within about ten minutes. Results from 40 2-hr runs (during half of which the pendulum was rotated 180°) showed no differential acceleration on the Cu-Pb masses from the water with a limit of±2.4×10−9 cm/sec2.Angular deflection measurements were based on a time-interval method developed previously by the author. Fiber drift was roughly constant during runs and always in the counterclockwise direction looking down on the apparatus, a result which may have arisen through interaction of the Coriolis force with vertical vibration of the pendulum. The principal error in measurement was from small discontinuous changes in the fiber drift-rate. The fiber motion suggested the presence of a small amount of second harmonic which was removed from the data by digital filtering. Direct measurement showed that pendulum tilt was a negligible source of systematic error. A 2-&sgr; limit was set on the “isospin coupling constant” of&agr;0=±0.001at&lgr;=100-m.©2001 American Institute of Physics.