Buildings with different mass levels were monitored in the summer of 1993 in Pala, South California, under different ventilation and shading conditions. The effect of mass in lowering the daytime (maximum) indoor temperatures, in closed and in night ventilated buildings, was thus evaluated. Night ventilation had only a very small effect on the indoor maxima of the low-mass building. However, it was very effective in lowering the indoor maximum temperatures for the high mass building below the outdoor maxima, especially during the ＇heat wave＇ periods. On an extremely hot day, with outdoor maximum of 38 degrees C (100 degrees F), the indoor maximum temperature of the high-mass building was only 24.5 degrees C (76 degrees F), namely within the comfort zone for the humidity level of California. Comment: In 1994 the monitoring has been continued, first with the original dark color of the envelope and then with the buildings painted white, as well as under natural, all-day ventilation with open windows. The results of the 1994 experiments will be reported in Part II.