We studied the functional effects of vasoactive intestinal peptide (VIP) and the structurally related peptide histidine isoleucine (PHI) in segments of bovine and human intrapulmonary artery. In both species, VIP caused nearly complete relaxation of precontracted vessel segments. The EC₅₀ was 1.3 ± 0.3 × 10^–9 M (mean ± SD) in bovine and 3.4 ± 0.4 × 10^–9 M in human pulmonary artery. The response to VIP was not endothelium-dependent and it was not affected by either adrenergic and cholinergic blockade or by cyclooxygenase inhibition. PHI also relaxed human and bovine vessels but this related peptide was significantly less effective than VIP. We conclude that VIP is a potent inhibitor of bovine and human pulmonary artery, which appears to act directly on vascular smooth muscle. These data support the concept that VIP may be a neurotransmitter which modulates pulmonary artery tone in both man and cow.