Prostaglandins of the A, E and F series administered to mature male mice at a dose level of 20 μg b.i.d. for 10 days significantly decreased serum testosterone (T) levels. In male rats, treatment with 150 μg PGA1 or PGA2 b.i.d. for 10 dayssignificantly reduced plasma (T) levels, testis weight and the weight of accessory reproductive organs. Similar effects of PGE2 and PGF2α, in male rats were previously reported. However, PGA, and PGA2, unlike PGE2 and PGF2α, did not alter plasma LH levels. The results provide further evidence that PGEs and PGFs canreduce plasma T levels, and indicate that concentration of T in the plasma of rats and mice can be decreased also by administration of PGA1, or PGA2.(Endocrinology95: 311, 1974)