The role of the α₁-adrenoceptor subtypes, and their possible change with maturation, in α₁-adrenoceptor-induced pressor responses in the rat has not been established. Thus, the effects of the α_1D-, α_1A/1D- and α_1B/1D-adrenoceptor antagonists, BMY 7378 (8-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl) 8-azaspiro (4.5) decane-7,9-dione 2HCl), 5-methyl-urapidil and chloroethylclonidine, respectively, on the pressor responses induced by phenylephrine in 1- and 5-month-old pithed rats were investigated. The pressor responses induced by phenylephrine were competitively antagonized by both BMY 7378 and chloroethylclonidine in 5-month-old, but not in young immature animals; in marked contrast, 5-methylurapidil antagonized with similar potency the phenylephrine-induced pressor responses in animals of both ages. The present pharmacological data suggest that functional expression of α_1D-adrenoceptors in the rat resistance vessels increases with age; α_1A-, but not α_1B- or α_1D-adrenoceptors, seem to predominate in immature animals. These findings represent the first evidence that age-related changes in functional α₁-adrenoceptor subtypes occur in the systemic vasculature in vivo. ©1997 Elsevier Science B.V.