A series of α 1-adrenoceptor agonists evoked concentration-dependent contraction of isolated guinea-pig spleen strips ( (—)- adrenaline > (—)- noradrenaline ⪢ L-phenylephrine > (—)-(4 aR,10 a)-3,4,4 a,5,10 a- hexahydro-6- methoxy-4- methyl-9- methylhio-2H- naphth[2,3-b]-1,4- oxazine methoxy-4- methyl-9- methylthio2H- naphth[2,3-b]-1,4- oxazine ( SDZ NVI 085) > cirazoline) , whereas indanidine, methoxamine, oxymetazoline and UK-14.304 were ineffective. (—)-Noradrenaline-induced contractions were inhibited by chloroethylclonidine (3 × 10 −6 −6 × 10 −5 M) and partially attenuated by SZL-49 (10 −7−10 −6 M), but remained resistant to (±)-isradipine (10 −9−10 −7 M). The contractions of the splenic strips were competitively antagonized by low concentrations of the α 1B-adrenoceptor-selective antagonist, spiperone (pA 2 8.05), but by relatively high concentrations of the α 1A-adrenoceptor-selective antagonists, (+)-niguldipine (pA 2 6.32) and 5-methyl-urapidil (pA 2 6.95). The affinities of subtype-selective antagonists determined at guinea-pig spleen α-adrenoceptors significantly correlated with p K i values at rat liver α 1B binding sites ( r=0.96) and pA 2 values at putative α 1B-adrenoceptors in rat aorta ( r=0.95), but differed from p K i values at rat cortical α 1A binding sites and pA 2 values at α 1A-adrenoceptors in rat vas defens. Also no correlation was obtained between antagonist affinities at guinea-pig spleen α-adrenoceptors and α 1C binding sites in rabbit liver. Thus, from the (1) potencies of agonists, (2) affinities of subtype-selective antagonists and (3) differential sensitivity of the contractions to α 1-adrenoceptor inactivating agents and their resistance to Ca 2+ channel blockade, the α 1-adrenoceptor mediating smooth muscle contraction of guinea-pig spleen can be best characterized as being of the B subtype.