A narrow field of view Rayleigh lidar has been constructed at Millstone Hill/MIT Haystack Observatory (42.6 ¿N, 71.5 ¿W) for observations of middle atmospheric temperatures throughout the diurnal cycle. During a 31.5 h measurement on 19--21 March 2001 a mesospheric thermal inversion layer was observed in both the night and day. It developed near 60 km in altitude, progressed downward at 0.40¿0.06 km/h, and had an overlying adiabatic lapse rate. The inversion amplitude correlated with the evolution of stratospheric gravity wave activity, although the mesospheric perturbations were too large to be due to conservative gravity-wave growth alone. The 24 h mean temperature profile shows no evidence of a residual inversion layer. ¿ 2001 American Geophysical Union