The pharmacokinetics and pharmacodynamics of furosemide were investigated after intravenous (i.v.), 1 mg/100 g body weight, and oral administration, 2 mg per 100 g body weight, to spontaneously hypertensive rats (SHRs) and deoxycorticosterone acetate-salt-induced hypertensive rats (DOCA-salt rats). After i.v. administration, the 8 h urinary excretion of furosemide/g kidney (397 versus 572 micrograms) was significantly lower and the non-renal clearance (5.78 versus 3.94 ml min-1 kg-1) was significantly faster in SHRs of 16 weeks of age than in age-matched control Wistar rats. This suggested that the non-renal metabolism of furosemide could be faster in SHRs of 16 weeks of age than in age-matched control Wistar rats, and this could be supported by the significantly greater amount of 4-chloro-5-sulphamoyl anthranilic acid, a metabolite of furosemide, excreted in 8 h urine as expressed in terms of furosemide (11.1 versus 4.79% of the i.v. dose) in SHRs. It could also be supported at least in part by a study of liver homogenate; the amount of furosemide remaining per gram of liver after 30 min incubation of 50 micrograms of furosemide with the 9000g supernatant fraction of liver homogenate was significantly smaller (40.4 versus 43.7 micrograms) in SHRs of 16 weeks of age than in age-matched Wistar rats. The greater metabolic activity of furosemide in liver may also be supported by the result that the amount of hepatic cytochrome P-450 (0.7013 versus 0.5186 nmol/mg protein) and the weights of liver (3.52 versus 2.93% of body weight) were significantly greater in SHRs of 16 weeks of age than in age-matched Wistar rats. After i.v. administration of furosemide, the 8 h urine output (9.93 versus 16.5 ml) and 8 h urinary excretion of sodium (1.21 versus 2.05 mmol) and chloride (1.37 versus 2.17 mmol) per gram of kidney in SHRs of 16 weeks of age were lower than those in age-matched Wistar rats, this could be due to the significantly smaller amount of furosemide excreted in 8 h urine per gram of kidney. After oral administration, the pharmacokinetics and pharmacodynamics of furosemide were not significantly different between SHRs and the control Wistar rats of 16 weeks of age. After i.v. and oral administration of furosemide, there were no significant differences in the pharmacokinetics and pharmacodynamics between DOCA-salt rats and control SD rats of 16 weeks of age except for the significantly lower urinary excretion of potassium per gram of kidney in DOCA-salt rats.(ABSTRACT TRUNCATED AT 400 WORDS)