To clarify the function of polymorphonuclear leukocytes (PMN) in spontaneously hypertensive rats (SHR) and the effect of beraprost sodium (BS) on these functions, we examined superoxide anion (O2-) production and adherent activity by PMN, as well as modification of these functions by BS ex vivo and in vitro. In study 1, we measured PMN functions in 4-week-old SHR and Wistar-Kyoto (WKY) rats. In study 2 (ex vivo), 14-week-old SHR received vehicle (n = 6) and BS (30 microg/kg/day [n = 6] and 100 microg/kg/day [n = 7]) once daily for 4 weeks. In study 3 (in vitro), PMN from 18-week-old SHR were incubated with BS (0.1 and 1 micromol/L) and theophylline (200 micromol/L), which is reported to inhibit the PMN O2- production. Systolic blood pressure, platelet counts, and PMN O2- production stimulated by phorbol ester myristate acetate were significantly elevated in 4-week-old SHR compared with WKY (P < .05). Beraprost sodium decreased the ex vivo PMN O2- production, serum superoxide dismutase activity, and platelet counts (P < .05); however, BS did not reduce the in vitro PMN O2- production. These data support our hypothesis that the enhanced PMN function contributes to the cardiovascular damages during the early phase of SHR, and that BS has merit for preventing the O2- related organ damages in this model.