The electrical responses of rat islet cells maintained in culture with varying levels of glucose were determined. After culture with 2.8 or 4.2 mM glucose, the beta-cells did not respond electrically to 27.8 mM glucose. However, after culture in 5.6 mM glucose, the cells responded to 27.8 mM glucose in short term experiments with depolarization and an increase in the incidence of spike activity. Islet cells maintained in 8.4 or 14.0 mM glucose had increasingly greater (negative) membrane potentials. The incidence of glucose-induced spike activity was the greatest for cells which had been cultured in 14.0 mM glucose. Raising glucose in the culture medium from 2.8 to 14.0 mM for the last 2 days of culture restored the ability of the islets to respond electrically to glucose in short term experiments. These observations show that the electrical phenomena of the beta cell membrane are affected by alterations in the level of glucose in the culture medium and serve as reliable indicators of functional glucose sensitivity of the islet cells.