The Floridan aquifer system consists of the Upper and Lower Floridan aquifers separated by a middle confining unit. The potentiometric surface of the Upper Floridan aquifer was mapped by determining the altitude of water levels in a network of wells and represented on a map by contours that connect points of equal altitude. This map represents water-level conditions near the end of the spring dry season when groundwater withdrawals for agricultural use were high. The cumulative rainfall for the study area was 4.84 inches above normal for the period from June 1992 to May 1993. Hydrographs of selected wells generally indicated that water levels in wells north of the Hillsborough/Pasco County line (northern area), where the aquifer generally is unconfined and groundwater withdrawals are relatively small, remained fairly uniform from year to year and season to season, whereas water levels south of the county line (southern area), where the aquifer is confined and withdrawals are relatively large, showed large year-to-year and seasonal fluctuations. Maximum daily water levels for selected wells from May 1992 through May 1993 showed that water levels were relatively stable during May 1993 in the northern part of the study area, but were declining rapidly in the southern part. Water levels in most wells measured in May 1993 were lower than those measured in September 1992. The greatest decline in water levels occurred in southern Hillsborough, central Manatee, and northern Sarasota Counties as a result of heavy seasonal groundwater withdrawals for irrigation. Water levels measured in May 1993 were generally higher than those reported for May 1992. The general increase in water levels during this period was largely the result of above-normal rainfall that resulted in below-average seasonal groundwater withdrawals from the aquifer.