Cytokines have been shown to influence susceptibility to febrile seizures and epilepsy. In this study, the role of interleukin-1beta (IL-1beta) was examined in developing rats. IL-1beta and interleukin-1 receptor antagonist (IL-1ra) were administered to developing rats, and seizures were induced by moist warm air. Twenty male Lewis rats (21-23 days old) were divided into two groups (IL-1beta and saline control groups) and two holes were made in the skull for EEG electrodes. We applied human recombinant IL-1beta intra-nasally 1h before seizures induced by moist warm air. The brain temperature at the appearance of seizure discharges on EEG, and the latency time from the hyperthermia onset until the appearance of seizure discharges on EEG were measured. And the same study using IL-1ra was performed. The median brain temperature for the IL-1beta group, 42.6 degrees C (range: 41.8-43.0), was significantly lower than that for the control, 42.9 (42.3-43.4) (P=0.043). The brain temperature for the IL-1ra group, 43.3 (42.8-43.7), was significantly higher than that for the control, 42.9 (42.2-43.5) (P=0.011), and the latency time for the IL-1ra group, 398s (270-561), was significantly longer than that for the control, 325 (252-462) (P=0.035). These results demonstrate that IL-1beta promotes hyperthermia-induced seizures in developing rats.