Glucose tolerance and insulin secretion were studied in non-diabetic littermates (n = 154) of BB diabetic rats, aged 4-6 months. Initial screening involved two intraperitoneal glucose tolerance tests (0.2 g/100 g body weight) performed one week apart. Nineteen rats (12%) were found to have impaired tolerance which persisted in 14 (74%) (group 1) and was transient in five animals (group 2). Seven rats progressed to overt diabetes in group 1, but none in group 2. Group 1 was characterized by (a) sustained abnormalities in glucose response to oral and intraperitoneal glucose, as well as intraperitoneal tolbutamide and arginine; (b) fasting hypoinsulinaemia; (c) decreased insulin response to glucose and tolbutamide; (d) suppression of the early and late phases of immunoreactive insulin response to intravenous glucose; (e) no systematic abnormalities in glucagon secretion; and (f) the presence of significant insulitis. The group 2 rats had (a) normal glycaemic response to oral and intraperitoneal glucose, tolbutamide and arginine on further testing; (b) normal fasting insulin but excessive insulin response to glucose and tolbutamide, but not to arginine, and (c) mainly normal islet morphology. Thus, impaired glucose tolerance may occur in BB rats with either hypoinsulinaemia or hyperinsulinaemia.