Isolated tail arteries from stroke-prone spontaneously hypertensive rats (SHRSP), but not from normotensive Wistar-Kyoto rats (WKY), exhibit oscillatory contractions in response to norepinephrine. Previous studies indicate that the mechanism for these oscillations involves altered membrane calcium and/or potassium handling, and that this vascular change is a genetic defect associated with hypertension in SHRSP. The purpose of this experiment was to determine whether treatment of SHRSP with the calcium entry blocker felodipine would alter oscillatory activity. Adult SHRSP and WKY rats were treated orally with felodipine for 8 weeks. Felodipine treatment produced a significant decrease in blood pressure in SHRSP (control SHRSP: 240 +/- 7 mmHg, n = 6; felodipine-treated SHRSP: 164 +/- 8 mmHg, n = 5, P less than 0.05; tail-cuff method). Helically-cut tail artery strips from all rats were mounted in tissue baths for isometric force recording and exposed to norepinephrine (6 x 10(-9) to 6 x 10(-6) mol/l) for 20 min at each concentration. Oscillatory activity was defined as the sum of the magnitudes of all phasic contractions occurring during the final 10 min of norepinephrine incubation. Oscillatory activity was markedly reduced in tail arteries from felodipine-treated SHRSP when compared with control SHRSP. Felodipine also inhibited oscillatory activity when added directly to the tissue bath. It seems, therefore, that felodipine may lower blood pressure in SHRSP, at least in part, by correcting the genetic defect responsible for oscillatory activity.