Expression of ovine kidney AT1 and AT2 receptor mRNA is developmentally regulated; AT1 mRNA levels increase during the last trimester of gestation while AT2 mRNA expression decreases after the second trimester of gestation. We sought to determine whether expression of AT1 and AT2 mRNA follows a similar developmental pattern in the ovine heart, where angiotensin (AII) has been postulated to have a number of functions. Using Northern blot analysis, we found that in contrast to the kidney, cardiac AT1 mRNA levels remain unchanged in all four chambers of the heart between 90 d gestation (term 145 d) and 2 mo of life. On the other hand, left ventricle AT2 mRNA levels were highest during fetal life, and significantly decreased (p<0.05) following birth (118±30 vs 46±13 net counts at 135 d gestation and 2 d of life respectively). Similar changes were seen in the other three chambers. To determine if the decrease in AT2 mRNA following birth is regulated by the increase in the activity of the renin-angiotensin system which normally occurs at this time, we studied the effects of AII infusion (10 ng/h) for 24 h in twin ovine fetuses (130 d gestation) on cardiac AT1 and AT2 mRNA expression. Plasma AII increased from 93±6 to 175±25 pg/ml(p<0.05) in AII-treated but not control fetuses. However, neither cardiac AT1 or AT2 mRNA levels were effected by AII infusion, whereas AT2 mRNA levels in renal cortex decrease in response to AII. These results demonstrate that the developmental changes in the expression of AT1 and AT2 mRNA are tissue specific. We speculate that the tissue-specific developmental patterns may reflect the different roles these receptors play within specific organs.