This study was designed to test the hypothesis that target-cell activity influences the degree and time course of interleukin 1β (IL-1β)–mediated β-cellimpairment in vitro. Functional and morphological studies were performed in cultured newborn rat islets of Langerhans exposed from 6 h to 6 days to 50–2000 ng/L recombinant human IL-1β. β-Cell activity was modulated by glucose and nonglucose agents (15 mM L-leucine and 10 μM of long-acting somatostatin analogue SMS 201–995). In 11 mM glucose, 2000 ng/L of IL-1β caused inhibition of insulin release after ∼6 h of exposure to IL-1β; in 3.3 mM glucose culture, onset of inhibition was delayed by this IL-1β concentration until after 48 h of exposure. Similarly, stimulation and suppression of β-cell function with L-leucine and SMS 201–995, respectively, resulted in acceleration and delay of IL-1β-mediated inhibition. The dose-response curve of the IL-1β effect was shifted left- and rightward during high and low β-cell activity, respectively. In analogy, increasing IL-1β concentration, exposure time, and β-cell activity resulted in increasing islet disintegration. Thus, the resting β-cell is more resistant to IL-1β-mediated impairment than the working β-cell.