Previous studies have shown that systemic hypoxia evokes vasodilatation in skeletal muscle that is mediated mainly by adenosine acting on A(1) receptors, and that the vasoconstrictor effects of sympathetic nerve activity are depressed during hypoxia. The aim of the present study was to investigate the role of adenosine in this depression. In anaesthetised rats, increases in femoral vascular resistance (FVR) evoked by stimulation of the lumbar sympathetic chain with bursts of impulses at 40 or 20 Hz were greater than those evoked by continuous stimulation at 2 Hz with the same number of impulses (120) over 1 min. All of these responses were substantially reduced by infusion of adenosine or by graded systemic hypoxia (breathing 12, 10 or 8% O-2), increases in FVR evoked by continuous stimulation at 2 Hz being most vulnerable. Blockade of A, receptors ameliorated the depression caused by adenosine infusion of the increase in FVR evoked by 2 Hz only and did not ameliorate the depression caused by 8% O-2 of increases in FVR evoked by any pattern of sympathetic stimulation. A(2A) receptor blockade accentuated hypoxia-induced depression of the increase in FVR evoked by burst stimulation at 40 Hz, but had no other effect. Neither A, nor A(2A) receptor blockade affected the depression caused by hypoxia (8% O-2) of the FVR increase evoked by noradrenaline infusion. These results indicate that endogenously released adenosine is not responsible for the depression of sympathetically evoked muscle vasoconstriction caused by systemic hypoxia; adenosine may exert a presynaptic facilitatory influence on the vasoconstrictor responses evoked by bursts at high frequency.