To determine precisely the influence of high blood pressure and age on the baroreflex sensitivity (BRS), we measured it in conscious genetically hypertensive (LH), normotensive (LN) and low-blood pressure (LL) rats of the Lyon strains. Groups of male rats were studied at the age of 5, 9, 13, 21 and 40 weeks. Using our previously described technique, their blood pressure (BP) and heart period (HP) were recorded beat by beat, in the conscious unrestrained state. Each animal received 2 or 3 i.v. injections of phenylephrine (PHE, 5 micrograms/kg) and nitroglycerin (NG, 100 micrograms/kg). The BRS (msec/mmHg) was computed as the slope of the closest relationship found between systolic BP (SBP) and HP changes. Otherwise, in the 13-week-old rats, BRS had been measured in basal conditions, after vagal blockade (atropine: 2 mg/kg i.v.) and after beta-adrenergic blockade (propranolol: 2 mg/kg) so as to determine the relative importance of vagal and sympathetic components of the baroreflex. In LN and LL rats, the BRS measured with PHE and NG increased markedly between 5 and 9 weeks of age and then remained stable in LN rats or decreased between 21 and 40 weeks of age in LL rats. In LH rats, BRS remained stable between 5 and 9 weeks of age and increased slightly up to 21 weeks of age. Significant differences between LH and LN rats were observed starting from 9 weeks of age when BRS was measured with PHE and only at 9 weeks of age when it was measured with NG injections. In 13-week-old rats, the cardiac baroreflex response was estimated to depend for 82 p. 100 and 18 p. 100 upon vagal and sympathetic components respectively when BRS was measured with PHE injections and of 68 p. 100 and 32 p. 100 when it was measured with NG injections.(ABSTRACT TRUNCATED AT 250 WORDS)