Previous studies on spontaneously hypertensive rats (SHR) have yielded inconsistent information about functional aberrations of the presynaptic alpha 2- and beta 2-adrenoceptor-mediated modulation of sympathetic neurotransmitter release. In the present investigation we studied the capacity of presynaptic beta 2-adrenoceptors that enhance noradrenaline (NA) release in the portal vein of freely moving, unanesthetized SHR and normotensive Wistar rats (WR) using the beta 2-selective agonist fenoterol. The results show that the presynaptic beta 2-adrenoceptor population in SHR responds to significantly lower dosages of fenoterol than that in WR. The reason for this enhanced action, however, could not be attributed to the beta 2-adrenoceptor itself, nor to a diminished neuronal uptake of NA, but to a diminished responsiveness of the presynaptic alpha 2-adrenoceptor. Stimulation of presynaptic alpha 2-adrenoceptors with oxymetazoline (45 micrograms/min) decreased basal NA levels by 46% in WR and by 3% in SHR. Blockade of alpha 2-adrenoceptors, using 0.5 mg/kg yohimbine, induced a 4.86-fold rise in the basal NA level in WR but only a 1.89-fold rise in SHR. A subsequent dose of fenoterol, however, resulted in a further 2.5- and 2.6-fold rise in WR and SHR, respectively, indicating that there is a normal presynaptic beta 2-adrenoceptor population in the vasculature of SHR.