Ontogeny of the vasorelaxant effects of atrial natriuretic factor (ANF) and isoprenaline and of the vasoconstrictor effects of potassium and phenylephrine were studied on aortic strips from rabbits aged 1, 2, 3, 6, 10 and 32-48 (adult) weeks. ANF caused complete relaxation of phenylephrine- and potassium-contracted aortic strips from all age groups of rabbits and its potency was not altered by age and by the agent used to cause contractions. On the other hand, the vasorelaxant efficacy and potency of isoproterenol decreased with age. Also, isoprenaline was less potent and efficacious on aortic strips contracted with potassium than on strips contracted with phenylephrine. The sensitivity of aorta to phenylephrine increased after 2 weeks of age, but that to potassium remained the same at all ages. In conclusion, our data indicate that the vasorelaxant effects of ANF and the vasoconstrictor effects of potassium on rabbit aorta are not altered by age whereas the sensitivity of the vascular smooth muscle to isoproterenol decreases and that to phenylephrine increases with maturation.