La Rance was the first tidal power plant worldwide with a capacity of 240 MW supplied by 24 bulb units working in both flow directions. Many tests had been undertaken in laboratory and on the prototype of Saint-Malo, but no real commissioning tests had been carried out until now. Measuring discharge at La Rance is indeed complex because of a very low and changing head from 2 m to 11 m depending on the tides, large gate bays of 8.7 m on 8.8 m, and high flow rate variations from 75 m3/s to 280 m3/s. It was decided to use current meters on a moving frame in the bulkhead slot for flow rate measurement. Further challenge was to maintain an almost constant head of 4 m during data acquisition. Full exploration of the velocity field was completed in both directions in 6 vertical positions. Additionally cam tests were performed, using index measurement with the frame in the middle position.