The objective of this study was to examine the effect of antihypertensive treatment on the structure of intramyocardial resistance vessels in spontaneously hypertensive rats. The rats were divided into four groups: one was used as control and the other three were treated from the age of four to 24 weeks with isradipine, hydralazine, and metoprolol, respectively. Half of the animals in each group were examined at the end of active treatment and the rest were examined three weeks later. The rats were anesthetized and killed during constant flow perfusion with 1 percent glutaraldehyde. The media index was determined by point counting. The media indices of rats treated with isradipine and hydralazine were significantly smaller than those of age-matched spontaneously hypertensive rat controls, whereas the media indices of rats in the metoprolol group did not differ significantly. Three weeks after treatment withdrawal, the media index tended to increase in all three groups, but the values for the isradipine and hydralazine groups were still significantly reduced. Non-invasive blood pressure measurements taken at the same time demonstrated a significant blood pressure reduction in all groups, although differences within each treatment group were evident. All pressures had stabilized on the level of spontaneously hypertensive rats three weeks after withdrawal. Thus, it is evident that both isradipine and hydralazine were able to prevent hypertrophy of intramyocardial vascular structure and continue to do so even after treatment withdrawal. This finding is consistent with previous findings, suggesting a close relationship between the extent of blood pressure reduction and the degree of prevention of vascular hypertrophy.