As a direct inhibiting effect of corticosterone (0.2 mg/l) has been shown upon 16.7 mmoles/l glucose-induced insulin secretion rate of rat islets of Langerhans, modulations of this inhibition were studied. The inhibitory effect was observed over a wide range of stimulatory glucose concentrations (8.3--33.4 mmoles/l), but not with non-stimulating glucose values (0-4.2 mmoles/l). During the steady-state insulin secretory rate induced by glucose 16.7 mmoles/l, the inhibiting effect needed about fifteen minutes to become statistically significant; it persisted as long as the glucose stimulation was maintained, independently of corticosterone presence. The inhibition during a subsequent incubation without corticosterone after a preincubation period with the steroid and glucose concentrations between 4.2 and 11.1 mmoles/l, appeared only if the glucose concentration during the preincubation period was a stimulating one. A minimal steroid presence time (10 min) appeared to be necessary for the induction of the subsequent inhibitory effect of corticosterone in presence of 16.7 mmoles/l glucose. Our data suggest that the inhibition of insulin secretion is constant over a wide range of glucose concentrations on condition that the hormone presence lasts more than ten minutes and glucose concentration is a stimulating one; then it persists as long as a stimulatory glucose concentration is provided.