We field-tested the Reproductive Index (RI) of the great blue heron (Ardea herodias) Habitat Suitability Index (HSI) model in the southcentral Great Plains with the aid of Geographic Information System (GIS) technology. From January 1993 through May 1994, populations of great blue herons in 18 rookeries located throughout Oklahoma were monitored, and GIS was used to evaluate data on rookery habitat structure and surrounding landscape features. Eighteen rookeries were classified as potential nest sites and RI ratings were determined for each rookery according to model criteria. The RI identified only 3 (17%) of the 18 rookeries as suitable habitat for reproduction. After modifying the RI using habitat and landscape data from the 18 rookeries in Oklahoma, the RI more reliably characterized the habitat suitability of rookeries in the southcentral Great Plains. However, it was not significantly related to initial size of the rookery population or its size at the end of the breeding season.