1. The subtype of alpha1-adrenoceptor mediating contractions to phenylephrine of the rat thoracic aorta, mesenteric artery and pulmonary artery were investigated by use of antagonists which show selectivity between the cloned alpha1-adrenoceptor subtypes in binding studies. 2. Cumulative concentration-contraction curves for phenylephrine were competitively antagonized in the rat thoracic aorta by prazosin (pA2 9.9), WB4101 (pA2 9.6), 5-methylurapidil (pA2 8.1), benoxathian (pA2 9.2) and indoramin (pA2 7.4). These compounds were also competitive antagonists in the mesenteric and pulmonary arteries (except for 5-methylurapidil in the pulmonary artery), (prazosin pA2 9.9 and 9.7; WB4101 pA2 9.8 and 9.6; 5-methylurapidil pA2 7.9 and pK(B) estimate 8.0; benoxathian pA2 8.8 and 9.3; indoramin pA2 7.2 and 7.5, respectively). 3. RS 17053 was not a competitive antagonist in any blood vessel as Schild plot slopes were greater than unity. The pK(B) estimates for RS 17053 were 7.1 in aorta, 7.0 in the mesenteric artery and 7.7 in the pulmonary artery. 4. The alpha1D-subtype selective antagonist BMY 7378 appeared to be non-competitive with shallow Schild plot slopes. The data were better fitted with two lines in all tissues, with Schild plot slopes that were no longer different from unity, except in the pulmonary artery. The higher affinity site for BMY 7378 in the aorta had a pA2 of 9.0, while it was 8.8 and 8.9 in the mesenteric and pulmonary arteries, respectively. 5. MDL73005EF acted in a non-competitive manner in all three blood vessels, with shallow Schild plot slopes. The pK(B) estimates for MDL73005EF were 8.4 in aorta, 7.5 in the mesenteric artery and 8.0 in the pulmonary artery. 6. In all three blood vessels the functionally determined antagonist affinity estimates correlated best with published pKi values for their displacement of [3H]-prazosin binding on membranes expressing cloned alpha1d-adrenoceptors compared with alpha1a- or alpha1b-adrenoceptors. The antagonist affinity estimates in the aorta, mesenteric and pulmonary arteries correlated highly with their previously published pA2 values in rat aorta (alpha1D) and less well with those for alpha1A- and alpha1B-adrenoceptors mediating contraction of the rat epididymal vas deferens and rat spleen, respectively. 7. The results of this study suggest that the contraction to phenylephrine of the rat thoracic aorta, mesenteric artery and pulmonary artery are mediated in part via the alpha1D-subtype of adrenoceptor. The data for both BMY 7378 and MDL73005EF in all three blood vessels are consistent with receptor heterogeneity. However, the identity of the second site is unclear.