The stable metabolites of thromboxane A2, prostaglandin E2, and prostaglandin F2 alpha (TxB2, PgEM, and PgFM, respectively) were measured in the blood plasma of nine castrated male pigs, each inoculated with 2 x 10(5) sporocysts of Sarcocystis miescheriana (group A), and in that of nine non-infected controls (group B). All infected pigs developed mild disease, the clinical signs being most severe between days 14 and 17 post infection (p.i.). In the infected pigs of group A, the TxB2 plasma levels increased with the onset of the acute phase of illness (12 days p.i.), reaching peak values at day 14 p.i. The mean TxB2 values were significantly higher in the infected pigs from day 12 p.i. until the termination of the experiment on day 21 p.i. The PgEM values increased steadily in the infected pigs from day 12 p.i. until day 21 p.i. but remained relatively constant in the control pigs during the same period. In contrast, PgFM values remained low in the infected pigs throughout the experiment, and no significant differences between infected and non-infected pigs could be found. We conclude that the elevated TxB2 and PgEM values reflect a major involvement of prostanoids in the pathogenesis of sarcocystiosis.