Plasma insulin levels were measured in 8 patients of whom 5 were surgically proven to have, and 3 had strongly suggestive evidence of, a functioning islet cell tumour. The fasting plasma insulin level was elevated in only 3 patients, despite repeated sampling. The administration of tolbutamide produced an excessive insulin response in 5 of the 8 patients, glucagon produced an excessive insulin response in 4 of 6 patients tested, 1-leucine produced a rise in plasma insulin in 1 of 2 subjects tested and the only subject studied after 50g. oral glucose had a high insulin response. The insulin response was highest after tolbutamide in all but one patient and this would appear to be the single most valuable test. However false negatives may occur and therefore the whole range of tests may have to be performed to establish the diagnosis. Two patients were studied during surgery and in one the insulin content of the tumour was assayed after extraction with acid alcohol. Four patients were studied again after treatment by surgery (2) and diazoxide (2) when normal insulin levels were found.