Measurements of the sum and difference of the travel times of acoustic pulses propagating in opposite directions provide powerful tomographic tools. Travel-time signals due to sound-speed perturbations cancel in the difference travel time, leaving only the much smaller signals due to currents. Reciprocal acoustic transmissions are particularly well suited to measuring large-scale barotropic flow and areal-average relative vorticity, both of which are of great oceanographic importance, but difficult to measure using other techniques. The use of reciprocal transmissions to measure ocean currents and vorticity has been tested in a series of experiments at steadily increasing ranges, from 25 to 1275 km. Acoustically derived currents and vorticity have been found to be consistent with independent measurements in all cases. The barotropic tides have provided the best test signals, because they are both well known and of large scale. Point measurements provided by current meters provide less stringent tests. The high-frequency travel-time fluctuations due to internal-wave-induced sound-speed perturbations have been found to largely cancel in the differential times, demonstrating that the ray paths of oppositely traveling signals are nearly reciprocal out to 1-Mm ranges, as is implicitly assumed when differential travel times are used to deduce ocean currents.