The integrity of the arterial baroreflex is central to cardiovascular homeostasis. There is evidence of altered cardiovascular regulation after acute traumatic brain injury (TBI). We hypothesized that arterial baroreflex is modified by acute TBI. An experimental study using 18 terminally anesthetized male Wistar rats weighing 240 to 260 g was undertaken at a university laboratory setting. Brain injury was induced using the lateral fluid percussion brain injury model. The fluid percussion device delivered an applied cortical pressure of 1.2 atm and 1.8 atm, producing mild and moderate TBI, respectively. Control animals underwent identical surgical procedures but no applied cortical pressure. Arterial baroreflex was assessed by determining the relationship between heart period (R - R interval) and systolic blood pressure using the modified phenylephrine pressor test adapted for the rat. The arterial baroreflex was tested before (Tcon), post-TBI, at 10 min (T10), and 30 min (T30). Analysis of baroreflex function after moderate TBI using repeated-measures analysis of variance revealed significant differences in baroreflex sensitivity (BRS) at T10 and T30 (F2,15 = 10.18; P = 0.005) compared with pre-TBI (weighted mean ± SD; Tcon, 0.39 ± 0.00 ms mmHg; T10, 0.85 ± 0.01 ms mmHg; T30, 0.81 ± 0.01 ms mmHg). The changes in BRS were not significant after mild TBI (P = 0.152). Repeated-measures analysis of variance comparing trends between the three groups indicated significant differences between the control and moderate TBI groups only (F2,15 = 6.26; P = 0.01). Acute TBI of moderate severity is associated with an early significant modification in arterial BRS. This is a key component of cardiovascular homeostasis. The clinical implications of this observation require further investigation.