A first analysis is presented of the San Marco 3 density data which were obtained on a continuous instantaneous basis by the 'balance' experiment during the period May 3 through November 2, 1971, in the latitude belt of ¿3.2¿. The analysis has been carried out in two ways: selecting constant altitude points (200--350 km) over a perigee revolution (24-hour local time variation) and reducing to the same altitude the data of the passes of single days. The main results for the diurnal variations are as follows. The Jacchia type R coefficient is strongly altitude dependent and increases between 200 and 280 km; R variation with UT is also apparent and may be related to the semiannual perturbation; and a significant content of 8-hour and 6-hour harmonics is present on the bulge profiles.