The aim of the study was to investigate to what extent inhibition of nitric oxide (NO) formation, cyclooxygenase and converting enzyme activities and vasopressin V1 receptors blockade affects the cardiovascular system in conscious, freely moving normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. The experiments were performed on 33 WKY and on 33 SHR. In series 1 (8 WKY and 8 SHR) animals received bolus injection of N omega-nitro-L-arginine - NLA (10 mg/kg), in series 2 (6 WKY and 6 SHR) bolus injection of indomethacin (10 mg/kg). In series 3 (8 WKY and 8 SHR) the animals received captopril as initial bolus (1 mg/kg) followed by constant infusion (1 mg/kg/min), in series 4 (11 WKY and 11 SHR) vasopressin V1 receptor antagonist 1-(1-mercapto-4-methylcyclohexaneacetic acid)-8-arginine-vasopressin (MeCAAVP) was infused (1.52 micrograms/kg/min). In series 1 in WKY NLA elicited a long-lasting, significant increase in mean blood pressure (max 46 +/- 3 mmHg at 40 min). In SHR mean blood pressure raises were not significant (max 22 +/- 6 mmHg). In series 2, both in WKY and SHR, indomethacin elicited only transient, nonsignificant increases in mean blood pressure. In series 3, the mean blood pressure fall during the captopril infusion was more pronounced in SHR than in WKY (45 +/- 2 mmHg vs 13 +/- 2 mmHg respectively). In series 4 vasopressin V1 receptor blockade caused a nonsignificant fall in mean blood pressure, both in WKY and SHR.(ABSTRACT TRUNCATED AT 250 WORDS)