The phasic contraction to phenylephrine of the rat isolated portal vein was investigated using functional studies. Phasic contractions to phenylephrine and caffeine could be produced after several minutes in Ca²⁺-free Krebs (solution) and were inhibited by cyclopiazonic acid or ryanodine. The phenylephrine and caffeine contractions were abolished within 10 min in Ca²⁺-free Krebs however and by nifedipine. This indicated the Ca²⁺ stores were depleted in the absence of Ca²⁺ influx through voltage-gated channels. The phasic contraction to phenylephrine was also abolished by niflumic acid even in Ca²⁺-free Krebs. This showed the response depended on intracellular Ca²⁺ release stimulated directly by depolarization resulting from opening of Ca²⁺-activated Cl^- channels, but did not require Ca²⁺ influx. In support of this, K⁺-induced phasic contractions were also produced in Ca²⁺-free Krebs. The phenylephrine but not K⁺-induced phasic contractions in Ca²⁺-free Krebs were inhibited by ryanodine or cyclopiazonic acid. This would be consistent with Ca²⁺ release from more superficial intracellular stores (affected most by these agents), probably by IP₃, being required to stimulate the phenylephrine depolarization.