Intracellular free calcium concentration, [Ca2+]i, of cerebral synaptosomes loaded by Fura-2/AM was measured by fluorescent method to clarify alterations of [Ca2+]i in infectious brain injury (IBI) in rats. The results showed that [Ca2+]i, water content (WC), and Evans blue content (EB) in groups injected by Bordetella pertussis suspension (BP) were increased compared to those in groups injected by normal saline (NS) at 0.5 h, 4 h, and 24 h, respectively. Increased [Ca2+]i in BP groups showed time-dependent relationship and the highest level in the 24 h group. There was positive correlation (0.5 h, 4 h) and no correlation (24 h) between [Ca2+]i and WC or EB. It is suggested that [Ca2+]i accumulation of cerebral neuronal cells plays an important role in the pathogenetic mechanism of the acute and delayed IBI induced by BP.