|Abstract||Both pre-injury and immediate post-injury administration of magnesium alleviates sensorimotor deficits following sensorimotor cortex damage. Furthermore, magnesium may alleviate sensorimotor deficits when administered weeks after sensorimotor cortex damage. The purpose of the present study was to examine the behavioral effects of magnesium administration at various time points after prefrontal cortex lesions in rats. Prefrontal cortex lesions produce various behavioral deficits that are thought to indicate cognitive dysfunction. In order to assess cognitive function, rats were tested on a spatial learning and memory test and a test of sensory attention. In the first experiment, rats received prefrontal cortex lesions. The rats were injected with a preoperative magnesium regimen (72, 48, and 24 hr), a postoperative magnesium regimen (15 min, 24 hr, and 48 hr), or saline. Three weeks later, the cognitive function of rats was assessed on the two behavioral measures. Results indicate that both preoperative and postoperative regimens of magnesium facilitated performance of lesioned rats on both behavioral measures. In the second experiment, rats were given prefrontal cortex lesions and were injected with either magnesium or saline on postoperative days 22¿28. Animals were tested beginning on postoperative day 22. Results show no effect of magnesium on either behavioral measure. These data suggest that early magnesium treatment may facilitate cognitive performance following cortical injury. However, the effectiveness of delayed magnesium treatment may be limited to sensorimotor deficits.