Several mutant mouse models for human diseases such as diabetes mellitus have been generated in the large-scale Munich ENU ( N-ethyl- N-nitrosourea) mouse mutagenesis project. The aim of this study was to identify the causal mutation of one of these strains and to characterize the resulting diabetic phenotype. Mutants exhibit a T to G transversion mutation at nt 629 in the glucokinase ( Gck) gene, leading to an amino acid exchange from methionine to arginine at position 210. Adult Munich Gck^M210R mutant mice demonstrated a significant reduction of hepatic glucokinase enzyme activity but equal glucokinase mRNA and protein abundances. While homozygous mutant mice exhibited growth retardation and died soon after birth in consequence of severe hyperglycemia, heterozygous mutant mice displayed only slightly elevated blood glucose levels, present from birth, with development of disturbed glucose tolerance and glucose-induced insulin secretion. Additionally, insulin sensitivity and fasting serum insulin levels were slightly reduced in male mutant mice from an age of 90 days onward. While β-cell mass was unaltered in neonate heterozygous and homozygous mutant mice, the total islet and β-cell volumes and the total volume of isolated β-cells were significantly decreased in 210-day-old male, but not female heterozygous mutant mice despite undetectable apoptosis. These findings indicate that reduced total islet and β-cell volumes of male mutants might emerge from disturbed postnatal islet neogenesis. Considering the lack of knowledge about the pathomorphology of maturity-onset diabetes of the young type 2 (MODY 2), this glucokinase mutant model of reduced total islet and total β-cell volume provides the opportunity to elucidate the impact of a defective glucokinase on development and maintenance of β-cell mass and its relevance in MODY 2 patients.