Wolfram syndrome, an autosomal recessive disorder, is caused by mutations in the WFS1 gene. WFS1 regulates endoplasmic reticulum (ER) stress and has a role in diseases involving ER stress as in case of β-cell death in diabetes. Valproic acid (VPA) is an anticonvulsant and mood-stabilizer. There are contradictory findings whether VPA increases insulin secretion (Luef et al., 2003) or changes its metabolism in liver (Pylvänen et al., 2006). In the study by Terasmaa et al. (2011) acute pre-treatment with VPA improved glucose tolerance in Wfs1 knockout (Wfs1KO) mice, but had no effect in wild-type (WT) mice. The aim of current study was to study insulin secretion and evaluate the effect of VPA in isolated pancreatic islets of WFS1-deficient mice. WT, heterozygous (Wfs1HZ) and Wfs1KO mice were used, 6 animals in each group. All studies were performed in male 5-6 months old mice. Pancreatic islets were isolated as previously described (Shimomura et al., 2009). Islets were preincubated for 1 h in Krebs-Ringer solution (KRBH) containing 2 mM glucose followed by 1 h at 37°C and 5% CO2 in selected assay solution (KRBH + 2 mM, 10 mM or 20 mM glucose or 200 µM tolbutamide and 2 mM glucose). Each assay media group contained 5 islets in duplicate per genotype. The supernatant was collected and insulin was measured with Ultra Sensitive Mouse Insulin ELISA Kit (Crystal Chem Inc., USA). To determine total insulin content, insulin was extracted using 95:5 ethanol:acetic acid solution (Shimomura et al., 2009). VPA was added to each assay solution with final concentration of 500 µM. Data are presented as mean ± SD. For comparisons ANOVA followed by Bonfrerroni post-test was used. A P value of <0.05 was considered statistically significant (P<0.05). WFS1-deficient mice had considerably less pancreatic islets than Wfs1HZ or WT mice (P<0.001) (Fig. 1). Wfs1HZ mice had also statistically fewer pancreatic islets compared to WT mice (P<0.001) (Fig. 1). Wfs1KO pancreatic islets secreted less insulin after stimulation with 2 and 10 mM glucose and with tolbutamide solution compared to WT and Wfs1HZ islets, but after stimulation with 20 mM glucose there was no statistically significant difference (P>0.05) (Fig. 2). Although VPA has been shown to increase insulin secretion in vitro (Luef et al., 2003), no such effect was seen in present study (Fig. 2). WFS1-deficient mice have impaired insulin secretion and may therefore be a diabetes model. Current study is in accordance with studies referring that VPA does not influence insulin secretion directly, but may have an effect upstream of insulin exocytosis.