Diurnal plasma glucose levels have been studied two hourly in streptozotocin diabetic rats during insulin treatment. Protamine Zinc Insulin induced a very steep fall in plasma glucose level from 359 +/- 100 (SD) mg/100 ml to 91 +/- 49 mg/100 ml within two hours. Plasma glucose was then low (13-60 mg/100 ml) until after 18 hours when an equally steep rise in glucose concentration ocurred. Six other insulin preparations and several insulin treatment regimens were tested with the aim of normalising the 24 hour plasma glucose profile of streptozotocin diabetic rats. One preparation, a non-commercial, very long acting Ultralente NOVO (Mc, ox pH 5.5) yielded a diurnal plasma glucose profile reasonably close to normal when it was administered once a day and when the dose was based on daily blood glucose measurements. Mean plasma glucose was 122 +/- 55 mg/100 ml with a nadir of 55 +/- 15 and a maximal of 187 +/- 99 mg/100 ml.