We previously demonstrated the presence of a long-acting antihypertensive factor (AHF) in the erythrocytes of essential hypertensive subjects (EHS). The present investigation demonstrates further that the AHF is also capable of producing a rapid and transient blood pressure lowering effect in stroke prone spontaneously hypertensive rats (from 26.8 +/- 1.7 to 20.1 +/- 1.5 kPa by 10-30 s, P < 0.001). An even more stronger antihypertensive effect could be found if the AHF prepared from normal subjects or rats was used. We also found that some hypertensive factor was present in EHS plasma, but not in normal subjects. The results of the present paper suggest that AHF deficiency and higher level of pressor substance may play an important role in the development of essential hypertension.