The relation between the development of hypertension and changes in behaviour was investigated. Open-field activity of male and female Spontaneously Hypertensive Rats (SHR) and Wistar-Kyoto controls (WKY) was scored at 4, 6 or 10 weeks of age. SHR generally showed higher locomotor activity and exploratory rearing behaviour, but lower grooming activity and defecation. These changes were found for rearing (3-5 fold increase) and grooming scores at all ages, ambulation at 4 weeks and 10 weeks (ambulation-inner) and defecation at 6 and 10 weeks of age. Differences were generally more pronounced in female rats. SHR showed less habituation than WKY. Already at the age of 4 and 6 weeks blood pressure was increased in SHR compared with WKY (approximately 120 mm Hg vs. 100 mm Hg). Between 6 and 10 weeks of age blood pressure increased rapidly in SHR, leading to a marked difference at the latter age (about 40 mm Hg), in both male and female rats. These experiments show that already at a young age, when blood pressure differences with WKY are small, marked behavioural changes are present in SHR. The altered behaviour could play a role in the development of hypertension in SHR.