Collagenase isolated mouse pancreatic islets were maintained in tissue culture for up to 5 months in a culture medium buffered with Hepes and the pH varying between 6.8 and 7.6. The amount of insulin released into the medium and the insulin response to glucose and glucose plus theophylline were measured during the culture period. It was found that islets cultured at pH 7.2 maintained the ability to release insulin into the medium for at least 5 months, which was longer than islets cultured at the other pH values. During the first weeks, the islets cultured at pH 7.6 had a higher response to both glucose and glucose plus theophylline than islets cultured at the other pH values, but later they lost their insulin releasing ability.