Increased serum levels of immunoreactive thromboxane B2 (iTXB2) were observed in spontaneously hypertensive rats of the Okamoto-Aoki strain (SHR) compared with normotensive Wistar-Kyoto rats (WKY). Serum iTXB2 levels in whole blood allowed to clot at 37 degrees C for 1 hour were significantly greater in SHR than WKY at 8, 16-20, and 38 weeks of age, whereas formation of iTXB2 by thrombin-stimulated whole platelets from 6 16-week-old SHR and 6 age-matched WKY was 399 +/- 44 and 377 +/- 38 ng/10(9) platelets/30 min, respectively. No significant difference in radioconversion of exogenous arachidonic acid to TXB2 was observed in whole platelets from SHR (18.2 +/- 2.5%, n = 4) and WKY (20.1 +/- 3.0%, n = 4) at 16 weeks of age. These results support the proposal that enhanced ability of blood from SHR to generate iTXB2 is independent of the stage of hypertension development. This enhancement probably depended on factors or blood elements other than platelets since no difference in formation was observed on stimulation of whole platelets.