The antioxidant activity of the hexane, ethyl acetate and methanol extracts of three varieties of sorrel (Hibiscus sabdariffa), as well as their potential for reducing blood viscosity, were studied in vitro. The samples screened were two red [(traditional red (TRED) and early bearing red (ERED)] and one white (WHTE) mature Hibiscus varieties. Antioxidant activity was measured by fluorescence spectroscopy using a model liposome system. Cyclooxygenase inhibitory activity was measured using cyclooxygenase (COX-1 and COX-2) enzymes by monitoring the rate of oxygen uptake in prostaglandin synthesis. Antioxidant activity was the highest overall in the red varieties compared to the white. The methanol extract from the three varieties had the highest activity. The methanol and ethyl acetate extracts of the three varieties of sorrel showed higher COX-1 enzyme inhibition than COX-2 and therefore had high potential to decrease blood viscosity. There was no COX-1 or COX-2 inhibition in the hexane extract. These findings support the ethnomedicinal use of H. sabdariffa in Africa and the Caribbean for the treatment of cardiovascular disease and hypertension.