The effect of glucose on growth of the beta cell population was characterized in rat pancreatic organ culture. The effect was monitored by measuring [3H]thymidine labelling indices of beta cells during the culture period and by quantitation of beta cell population size at the end of the culture period. Foetal and neonatal pancreases and different glucose levels were compared. Glucose was found to be effective in stimulating the beta cell proliferation and beta cell population increase at 300 mg/100 ml in 18-day foetal pancreatic explants, but not in 3-day neonatal explants, when compared to the control level of 100 mg/100 ml. A higher level of glucose (500 mg/100 ml) was ineffective and may even inhibit beta cell population growth. The higher than control levels of glucose (300 mg/100 ml and 500 mg/100 ml) were able to stimulate insulin secretion in neonatal tissue, but not in foetal tissue, although foetal tissue may develop such response later in culture. These results suggest that glucose stimulates beta cell proliferation and insulin secretion through different mechanisms. They further show that the potentiality for beta cell proliferation under glucose stimulation decreases with age of the explants and that the capacity for beta cell to proliferate as a function of glucose stimulation is limited.