Laser radar observations of the atmospheric sodium layer, made by various groups of workers during the past decade, have shown that the layer exhibits considerable time-varying structure. In this paper we describe horizontally spaced measurements of this structure and use the results of these observations to derive horizontal velocities in the height range 82 to 99 km. The measurements were made with a steerable lidar operating at a fixed zenith angle of 20.9 ¿ and pointed at each of three azimuths in turn, the complete sequence taking 5 min. The azimuths were chosen such that the intersections of the lidar beam with the sodium layer formed a triangle with approximately 60 km sides. We present here the analysis of a 10-hour data run taken on the night of July 21, 1979. After low pass filtering to remove rapid uncorrelated fluctuations, a cross correlation analysis was carried out for each of the height intervals in the range 82 to 99 km. The velocities calculated from the derived time shifts were found to vary with height in an oscillatory manner, with amplitude increasing with height. The east-west component showed a vertical wavelength of approximately 10 km, the north-south component a wavelength of 5 km, and the magnitude of the velocity varied between 6 and 146 m s-1.