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We studied the function of the cardiac sympathetic nerve varicosity in isolated right atrial preparations of spontaneously hypertensive rats (SHR) and Wistar- Kyoto (WKY) rats at 4, 9, 14, 20 and 50 weeks of age. Cumulative concentration-atrial period response (C-R) curves to isoprenaline showed similar maximum response and sensitivity (EC50) at all ages but there was an age-related fall in resting atrial rate. Similar results were found for methoxamine (α1- aradrenoceptor agonist) although the maximum response was significantly less than for isoprenaline. The time-dependent recovery (T1/2) of the fall in atrial period in response to sympathetic nerve stimulation from electrical field pulses (1-32 at 1 Hz) was enhanced by neuronal uptake inhibition by desipramine (0.1-1 µmol/l), to a similar degree at all ages. Pre-junctional α2- adrenoceptor stimulation by clonidine caused progressively more inhibition of the number of field pulses-fall in period relationship with age. SHR atria were similar to WKY rat atria at all ages except for a further impairment of the development of pre-junctional α2- adrenoceptors. These studies indicate that the function of the cardiac sympathetic varicosity matures early (by 4 weeks) and overall there is very little impairment in SHR versus WKY rat atria.