The formation of brain edema and subsequent intracranial hypertension are major predictors of unfavorable outcome following traumatic brain injury (TBI). Previously, we reported that arginine vasopressin (AVP) receptor antagonists reduce post-traumatic and post-ischemic brain edema in mice. The aim of the current study was to investigate further the contribution of arginine vasopressin V1a receptors to TBI-induced secondary brain damage in V1a receptor knock-out mice. V1a receptor knock-out (V1a -/-) and wild-type mice were subjected to controlled cortical impact (CCI), and edema (brain water content measured before and 24 h after CCI), primary and secondary contusion volume (15 min and 24 h after CCI), neurological function (one day before and seven days after CCI), body weight (before and seven days after CCI) and mortality were measured. Twenty-four h after CCI, V1a receptor knock-out mice had significantly less brain water content than wild-type mice (mean±standard error of the mean: 79.8%±0.3 vs. 80.6%±0.2, respectively), and secondary contusion volume was significantly smaller (38.2±1.7 mm(3) vs. 45.1±1.5 mm(3) in wild-type mice). Furthermore, the V1a receptor knock-out mice had less neurological dysfunction (3.2±0.8 vs. 7.0±1.4 in wild-type mice) and weight loss (1.0±1.0% vs. 4.9±1.8% in wild-type mice) seven days after CCI. Our data show that mice lacking V1a receptors have less secondary brain damage following experimental traumatic brain injury. We therefore conclude that V1a receptors may represent a novel drug target for preventing post-traumatic brain edema.