An analysis of a long series of lidar measurements of the vertical distribution of atmospheric sodium shows the existence of a long-term trend in the centroid height of the layer. After making allowance for the sampling effects of the mean seasonal and diurnal variations in centroid height, it is found that the height of the layer fell by approximately 700 meters between 1972 and 1987. A regression analysis indicates a mean rate of fall of 49¿12 m yr-1, with a correlation coefficient of -0.33, significant at the 99.99% level. The observed change is consistent with long term trends in mesospheric temperatures detected by other techniques. ¿ American Geophysical Union 1992