Concentration-dependent positive chronotropic response to phenylephrine (Phe) mediated by alpha 1-adrenoceptor in isolated normotensive and SHRsp rat right atria was determined under normal or depressed beat rate by stimulation of cholinergic M-receptor. The spontaneous beat rate was increased from 223 +/- 18 to 271 +/- 16 bpm by Phe 100 mumol.L-1 in the presence of propranolol 1 mumol.L-1. When the basal rate was lowered by stimulation of cholinergic M-receptor with carbachol, PE-induced increment of beat rate was enhanced to 101 +/- 23 bpm, accompanied by an increase of the force of contraction from 45 +/- 14% to 125 +/- 56% (vs control response). However, beta-adrenoceptor mediated positive chronotropic effect was attenuated in the presence of carbachol, and no positive inotropic effect was detected. PE-induced increase in beat rate in SHRsp (31 +/- 9 bpm) was lower than that in WKY (41 +/- 10 bpm). After pretreatment with carbachol, Phe caused increments in beat rate in SHRsp and WKY were 70 +/- 18 bpm and 114 +/- 17 bpm, respectively. It is suggested that in SHRsp, the function of cardiac alpha 1-adrenoceptor is lower than that in WKY.