The effects of chronic treatments of adult (aged 16-17 weeks) spontaneously hypertensive rats (SHRs) with different doses of gamma-linolenic acid (GLA) on blood pressure, heart rate, and body weight were studied. Twice-daily injection of SHRs with GLA lowered systolic blood pressure from 175 +/- 4 to 145 +/- 4 mm Hg within 1 week; systolic blood pressure in all three treated groups became stabilized in the normotensive range after 2 weeks of treatment. Control SHRs injected with olive oil showed only a transient decrease in systolic blood pressure on the third day. Heart rate and body weight were not affected by GLA treatment. Withdrawal of GLA treatment resulted in a rapid rise in systolic blood pressure within 1 day from 140 +/- 3 to 165 +/- 3 mm Hg, and it stabilized after 1 week at 191 +/- 5 mm Hg in the three experimental groups. A rapid increase in systolic blood pressure from 175 +/- 5 to 203 +/- 5 mm Hg was also observed in the control group treated with olive oil 1 day after the withdrawal of the treatment. Addition of aspirin (3 mg/kg) with the GLA treatment in olive oil abolished the antihypertensive effect of GLA. In contrast, once-daily treatment with GLA also lowered systolic blood pressure of the SHR, but blood pressure was still in the hypertensive range (170 +/- 6 mm Hg). Systolic blood pressure of control SHRs treated with olive oil was not affected. Plasma from untreated SHRs contained a small amount of GLA. One hour after the injection, the plasma level of GLA increased. We conclude that GLA when given twice daily is an effective antihypertensive agent in the SHR.