Cardiac alpha- and beta-adrenoceptor sensitivities were examined after chronic pretreatment of rats with reserpine. Increases in sensitivity would indicate that the receptor is under the influence of the sympathetic innervation, removal by catecholamine depletion with reserpine of the tonic effect of neurotransmitter release would permit receptor upregulation. The positive inotropic responses of paced left atria and papillary muscles and the positive chronotropic responses of spontaneously beating right atria were recorded. A concentration-response curve to isoprenaline (beta-adrenoceptor-mediated) was followed, in the presence of beta-blockade, by one to methoxamine (alpha-adrenoceptor-mediated). Methoxamine exerted positive inotropy of left atria and papillary muscles, the maxima being 43.2 +/- 2.7 and 26.8 +/- 4.4% of the isoprenaline maxima. A small positive chronotropy (16.5 +/- 5.6% maximum) of right atria occurred. After pretreatment with reserpine (1.0 mg kg-1 i.p. daily) for 7 days, the three preparations displayed supersensitivity to isoprenaline, revealed as a significant displacement (P less than 0.05) of the concentration-response curves to the left of those for control rats. Reserpine pretreatment, however, had no effect on the sensitivity to methoxamine. The increase in beta-adrenoceptor sensitivity to isoprenaline after reserpine pretreatment was accompanied by a significant 41.3% increase (P less than 0.05) in the number of [3H]-dihydroalprenolol [( 3H]-DHA) binding sites (Bmax) in ventricular membranes, although the dissociation constant (KD) was unaffected. There were more alpha-adrenoceptor [3H]-prazosin binding sites in ventricular than atrial membranes. However, there was no difference in KD or Bmax between reserpine-pretreated and control tissues.(ABSTRACT TRUNCATED AT 250 WORDS)