In a model of closed head injury in rats, a calibrated weight drop device was allowed to fall onto the skull's convexity over the left hemisphere 1 to 2 mm lateral from the midline. Prostaglandin (PG) levels were determined in the frontal cortex region remote from the site of injury where no macroscopic damage could be seen. Differential patterns of temporal changes were evident for PGE2, PGD2, thromboxane (TX) B2, and 6-keto-PGF1 alpha in the contused hemisphere, but no changes were found in the contralateral hemisphere. The major changes in PG levels were increased levels of PGD2 and 6-keto-PGF1 alpha that persisted from 18 hours until 10 days after injury. The ratio between TXB2 and 6-keto-PGF1 alpha, which reflects the vascular tone, increased during the early postinjury period (15 minutes and 1 hour) and decreased later, up to 10 days. Thus, a sustained imbalance in favor of the vasodilator is apparent; this may suggest an improved blood supply to the region. Both PGD2 and PGI2 have protective effects in the brain. We suggest that their endogenous increase may be part of a repair mechanism at the periphery of the injured zone.