It is believed that sodium photochemistry is strongly influenced by mesospheric atomic oxygen and ozone, and one might thus expect sodium variations to occur during twilight. We have investigated this expectation by measuring the sodium density using a laser radar system. We report the results for 31 evening twilights and compare them with recent model calculations. The measurements show a rapid decrease in sodium density at the bottom of the layer, immediately after sunset, in agreement with theoretical predictions. Na variations are triggered by NaOH photolysis, while Na airglow variation occurs from O3 change.