A major objective of the Gulf Coast Regional Aquifer-System Analysis is to use digital models of regional groundwater flow systems to develop better understanding and to enable better management of the resource. Modeling is the best available tool to synthesize most of the known information about the aquifer systems and to test hypotheses about the relative importance of the components of the systems. The 290,000-square-mile study area in the Gulf of Mexico Coastal Plain includes the Mississippi embayment, the Gulf Coastal Plain of Texas, and offshore areas beneath the Continental Shelf that are underlain by deposits of Tertiary and younger age which contain fresh and saline water. A 10-layer, finite-difference, variable-density model, with blocks 10 miles on a side, was used to simulate groundwater flow before development (about 1900) and in 1980, assuming steady-state conditions for both, and transient conditions from 1935-1980. Preliminary results indicate that the major factors controlling predevelopment regional flow are the topography and the outcrop pattern and geometry of aquifers and confining units. Geologic structure and the distribution of precipitation were less significant factors. Regional recharge areas include the hills east of the Mississippi River Alluvial Plain and areas in a band! parallel to and about 75 miles inland from the coast where the distance between major rivers is largest. Major regional discharge areas are the low-lying, nearly flat Mississippi River Alluvial Plain, the coastal lowlands, and major river valleys. The density of saline water in the deeper parts of the aquifer system probably has a substantial effect on regional groundwater flow and this effect extends even into the freshwater part of the system. Variable water density resulting from temperature and salinity variations may be a significant driving force that transports saline water from salt formations great distances in many directions, including updip. The distribution and rates of regional recharge and discharge have been substantially changed by development. Groundwater pumpage in 1980 was about five times the value of predevelopment regional recharge. About 80 percent of the pumpage was being supplied from increased regional recharge, with lesser amounts from decreased natural discharge and aquifer storage. Simulation of 1980 conditions, assuming no change in storage in the aquifer system, resulted in a reasonable match of simulated and measured heads. Results also indicate that resistance to vertical flow caused by many fine-grained beds within the permeable zones can be as important as resistance caused by regional confining units.