Survival and function of rat pancreatic b-cells are best preserved by culture at 10 mM glucose (G10) and deteriorate after a few days at low (G2-G5) or high (G30) glucose. To investigate the mechanisms by which glucose affects the b-cell phenotype, we measured gene mRNA levels in male Wistar rat islets precultured for 1 wk in G10 and further cultured 18 h in G2, G5, G10 or G30 (Affymetrix Rat Genome 230 2.0 oligonucleotide arrays, n=4). GCOS software (Affymetrix) was used to calculate match/mismatch probe signal intensities and assess differences in mRNA abundance, expressed as absolute signal log₂ ratios (SLR). From the 31099 probe sets on the arrays, 18081 were reliably detected in at least one glucose concentration. The SLR for at least one of the comparisons between the different glucose concentrations was >= 2, >= 1 and >= 0.5 in 170, 1039 and 3530 probe sets respectively. Using Self-Organizing Map cluster analysis, 3120 of the 3530 affected probe sets were classified in 6 clusters with monophasic concentration-dependencies that differed in overall direction (increase/decrease) and glucose threshold. The remaining 410 probe sets were grouped into 5 small clusters with complex mRNA expression profiles (V-shaped or inverted V-shaped with minimum or maximum in G5 or G10). Analysis of gene function in each cluster using GenMapp 2 suggested the presence of co-regulated metabolic and signalling pathways in rat islets (glycolysis, cholesterol synthesis, unfolded protein response, integrated stress response...). In conclusion, assessment of the islet mRNA abundance after 18 h exposure to a large range of glucose concentrations reveals at least 11 distinct clusters of concentration-dependent co-regulated genes. Our results will help identifying key molecules that maintain the differentiated b-cell phenotype as well as those responsible for b-cell deterioration after exposure to low or high glucose concentrations.