Myogenic properties of posterior cerebral arteries from normotensive and hypertensive rats were analyzed in vitro and quantified in terms of both pressure range limits and degree of myogenic activity. Spontaneously hypertensive rat (SHR) vessels were significantly narrower in a fully relaxed state, and both wall thickness and wall-to-radius ratios were increased. After equilibration in 1.6 mM calcium physiological saline solution a substantial tone developed which resulted in average diameter decreases of 34 and 37% in Wistar-Kyoto (WKY) and SHR, respectively; average lumen diameters were approximately 125 micron. Rapid changes in transmural pressure (delta P 10-25 mmHg/s) were applied and diameter responses measured continuously. Myogenic responses began 1-3 s after a change in transmural pressure, and arteries regained their initial diameters after a pressure step in about 2 min; a final, steady-state diameter was achieved in 4-5 min. Myogenic pressure ranges were 49-145 mmHg in WKY and 64-181 in SHR; when responses were segregated according to positive and negative pressure steps, more myogenic responses were observed at lower pressures for pressure step decreases when compared with pressure step increases. Thus myogenic ranges for increasing pressure steps were 71-151 (WKY) and 72-188 mmHg (SHR) and for decreasing steps 45-117 (WKY) and 57-148 mmHg (SHR). Myogenic responses in SHR were weaker than in WKY rats: the former maintained essentially a constant diameter over a wide range of pressures, whereas arteries from the latter decreased diameter with increasing pressures.(ABSTRACT TRUNCATED AT 250 WORDS)