The purpose of our study was to determine in rats the dependence of constrictor responses of intracerebral (parenchymal) penetrating arterioles on extracellular calcium. Vessel diameter was monitored in vitro, after spontaneous tone had developed. When the organ bat solution was changed from a physiological solution to a Ca2+-free one containing 0.5 mM EGTA, these vessels showed rapid and marked vasodilation to a mean +/- SEM of 159.8 +/- 8.2% of control (p = 0.018). After 3 minutes of pretreatment with Ca2+-free solution, constrictor responses to solutions of 140 mM KCl, pH 8.0, or 10(-4) M prostaglandin F2 alpha, were completely abolished, except for small transient contractions with the latter (6.6 +/- 1.8%). When vasoconstrictors were applied simultaneously with the Ca2+-free solution, all vessels still dilated to 159.8 +/- 8.4% (KCl), 146.7 +/- 6.1% (pH 8.0), and 124.3 +/- 9.1% (prostaglandin 2 alpha) of control (p = 0.018). Our results suggest that both vasomotor tone and constrictor responses in intracerebral arterioles are highly dependent upon activation from an extracellular calcium source.