The central United States, which is dominated by agriculture, has been selected as the first North American Carbon Program intensive campaign. Data sets that describe spatiotemporal variability in carbon fluxes are needed to support this campaign. Here we report the behavior of county cropland net primary production (NPP) in the first intensive region derived using USDA information together with crop-specific parameters that convert agronomic data into carbon fluxes. Total cropland area in the eight-state region was ~550,000 km2 (40% of total area), with some interannual variability but no temporal trend from 1972 to 2001. Regional production (P) was 0.3 Pg C yr-1 in the late 1990s, roughly 64% of the total US crop production. P was highest in the central counties (>1.2 Tg C yr-1). In contrast to area, both NPP (flux per unit area) and P (spatially aggregated flux) increased during the study period (46 and 51%, respectively). Corn was the dominant crop type grown in the region, contributing 58% of the total production, with soybeans second most productive but substantially less (20%) despite similar harvested area. Maximum year-to-year variability in P was high, generally greater than 30% for most counties, though exceeding 80% for some counties.