The possible existence of stable warming at middle altitudes in the martian atmosphere was first examined by J. R. Barnes (J. Geophys. Res. 95, 1401, 1990) in one-dimensional modeling of the effect of breaking gravity waves on the general circulation. This work was motivated by infrared observations of D. Deming, M. J. Mumma, F. Espenak, T. Kostius, and D. Zipoy (Icarus 66, 366, 1986) which suggested temperatures far from radiative equilibrium above polar regions at solstice. New observational evidence was obtained from heterodyne measurements of the J = 2-1 ¹²CO and ¹³ CO transitions made during the 1988 opposition. At this date, corresponding to southern summer solstice, these lines were mapped as a function of the position on the martian disk with the IRAM 30-m telescope. Wind velocities near 50 km altitude and vertical thermal profiles were derived from the Doppler shift and the line profiles, respectively, at various locations on Mars. The northern (winter) hemisphere profiles indicate the presence of a warm layer in the 40-60-km altitude range, with a relative warming of ≈5 K compared to smoothly decreasing temperature profiles such as those found in the southern hemisphere. This may be interpreted in terms of a temperature inversion in the northern middle atmosphere. Such a phenomenon may be similar to the stable inversion layers observed in the terrestrial mesosphere, which are believed to be a response to the gravity wave activity. We show that a zonally averaged model of the martian middle atmosphere, using the parametrization of gravity waves effects propounded by R. S. Lindzen (J. Geophys. Res. 86, 9707, 1981). is able to produce both a plausible zonal wind field consistent with the measurements of E. Lellouch, J. J. Goldstein, S. W. Bougher, G. Paubert and J. Rosenqvist (Astrophys, J. 383, 401, 1991) and a thermal field in good agreement with the observations. The 2-D modeling further indicates that weak equatorial thermal inversions may occur, due to the penetration of the summer hemisphere retrograde jet into the winter hemisphere.