In healthy male dogs, peripheral plasma testosterone (T), prostaglandin E-2 (PGE(2)) and seminal plasma PGE(2) levels were measured before, during and after ejaculation, and semen quality was examined after oral administration of PGE(2). Plasma T and PGE(2) levels did not change during these periods, but the seminal plasma PGE(2) level of combined the first and second fractions was significantly higher than those at 0-5 and 5-10 min after the start of ejaculation of the third fraction. Semen volume but not quality increased after PGE(2) administration. In conclusion, large amounts of PGE(2) are released from the prostate gland during the early part of ejaculation, and PGE(2) plays an essential role in secretion of seminal plasma.