It is currently accepted that young spontaneously hypertensive rats (SHR) have lower pressure levels than adult SHR in which the hypertension is well established, reaching the highest plateau at about 24 weeks, and that treatment with magnesium initiated during intrauterine life postpones the onset of cardiovascular alterations in SHR to about 90 days. These animals also show many behavioral alterations. The anxiety of SHR was measured in the elevated plus-maze, considering the age of the animals and previous ingestion of food supplemented with 1% magnesium oxide. Both young (1.5-2 months) and adult (5-6 months) SHR showed higher mean ( +/- SEM) percent of entries (48 +/- 3 and 51 +/- 3, respectively) and a longer mean ( +/- SEM) percent of time spent (43 +/- 5 and 55 +/- 5, respectively) in the open arms when compared to the mean ( +/- SEM) percent of entries and time spent in the open arms of young (35 +/- 3 and 20 +/- 4, respectively) and adult (36 +/- 7 and 17 +/- 5, respectively) normotensive Wistar rats. Treatment with magnesium oxide did not alter the performance of SHR in the elevated plus-maze. SHR showed an anxiolytic-like behavior which was neither influenced by age nor by antihypertensive treatment.