Five cell lines have been derived from a rat transplantable islet cell tumour using two different methods. The lines differ in morphology and contain and release different amounts of insulin and glucagon (insulin content, 1–90 pmol/106 cells; insulin release, 6–250 pmol/106 cells per 24 h; glucagon content, < 0·005–35 pmol/106 cells; glucagon release, < 0·05– 10 pmol/106 cells per 24 h). All the lines responded to the presence of the secretagogues leucine (20 mmol/l) plus theophylline (5 mmol/l) by increasing the rate of release of insulin approximately twofold. A high extracellular concentration of potassium (40 mmol/l) caused a three- to tenfold calcium-dependent increase in release of insulin and a parallel release of glucagon. Increasing the concentration of glucose from 2·8 to 16·7 mmol/l did not alter the rate of insulin release by any of the cell lines. J. Endocr. (1986) 109, 193–200