METROMEX, a field project designed and now in progress at St. Louis, involves 4 research groups planning and working cooperatively to study inadvertent weather modification by urban-industrial effects, and, in particular, man-made changes of precipitation. Urban areas affect most forms of weather and some, such as winds, temperature, and visibility, are obvious and their changes are easily measured. Inadvertent precipitation changes are harder to measure, and except for the well-documented La Porte anomaly, urban-related rain changes have had only limited study. Examination of historical data at St. Louis has revealed summer increases in the immediate downwind area of: 1) rainfall (10–17%); 2) moderate rain days (11–23%); 3) heavy rainstorms (80%); 4) thunderstorms (21%); and 5) hailstorms (30%). METROMEX field measurements in the summer of 1971 involved 220 raingages and hailpads, 3 radar sets, 70 rainwater collectors, 14 pibal stations, 4 meteorological aircraft, unique atmospheric tracers, and a wide variety of standard and unusual meteorological equipment. These measurement tools were used to provide information on 1) the processes of cloud and precipitation formation, 2) the chemistry of aerosols and rainwater, 3) the urban heat budget, 4) the 3-D patterns of precipitation elements, and 5) the airflow and cloud development for numerical models.