Diabetes of varying severity was induced in a colony of rhesus monkeys by streptozotocin injection or total pancreatectomy. Peripheral glomerular basement membrane thickness was determined using a reproducible and efficient procedure. Four groups of monkeys formed on the basis of sex, age, severity, and duration of treatment were analyzed. In control animals peripheral glomerular basement membrane thickness increased with age from 3 to 7 years, whereas monkeys aged 11 to 18 years had relatively constant measurements. Mean values did not differ between control and age-matched non-insulin-dependent streptozotocin-injected animals treated up to 13 years previously. Insulin-dependent streptozotocin-treated or pancreatectomized monkeys with a treatment duration of 10 to 27 months were not different from age-matched controls, whereas insulin-dependent monkeys treated 2 to 12.5 years earlier showed significantly increased peripheral glomerular basement membrane thickness. The peripheral glomerular basement membrane thickening observed in the insulin-dependent monkey is easily measured and is similar to changes observed in human diabetes mellitus.