We determined the amounts of prostaglandin (PG) D₂, PGE₂, PGF₂α, 6ketoPGF₁α and thromboxane (TX) B₂ formed from endogenous arachidonic acid (AA) or exogenous PGH₂ in the cochlear homogenates of 1-, 2-, 3-, 4- and 8-week-old Mongolian gerbil. After incubation at 37°C for 5 min, in 1-week-old animals 6ketoPGF₁α was determined by radioimmunoasssy to be the major PG formed from endogenous AA followed by PGE₂, PGF₂α and PGD₂. However, the formation of PGE₂ increased 4-fold in the course of maturation of animals to 8 weeks after birth. When we determined the amounts of PGs produced in the cochlear homogenate in the presence of AA exogenously added, there was no acceleration of each PG formation, while the activities of PGE₂-, PGF₂α- and PGD₂-synthetases increased about 7-fold from 1 to 8 weeks after birth.