The biosynthesis of prostaglandins (PGs) from the endogenous and exogenous precursor, arachidonic acid (AA), in renal papilla, medulla and cortex from neonatal to adult rats was investigated. Rat renal papilla and medulla incubated in the presence of [1-¹⁴C]AA released radioactive PGE₂, PGF_2α and PGD₂ which increased with age. No radioactive prostaglandins were found in the supernatants of renal cortex at any age studied. The amount of total prostaglandins released from the endogenous precursor also increased from 10 to 70 days of age, PGD₂ being the prostaglandin that showed the most important rise. In the cortex, only PGE₂ release increased with age. Cyclooxygenase (COX) activity was measured in papillary, medullary and cortical homogenates by using [1-¹⁴C]AA as substrate. Papillary and medullary COX activity increased after 10 days of age and continued to rise up to day 30 thereafter remaining unaltered until adulthood. Cortical COX activity was very low and decreased with age. These findings indicate the low capacity of the neonatal rat kidney to synthesize PGs.