OBJECTIVE: To determine whether renal interleukin-6 was produced constitutively under the normal physiological conditions and to evaluate the influence of hypertension development on interleukin-6 production in two different hypertensive models, the spontaneously hypertensive rat (SHR) and the two-kidney, one clip (2-K,1C) hypertensive rat. DESIGN: In a chronic study, Wistar rats and SHR, aged 4, 5, 7 and 9 weeks, and 2-K,1C Goldblatt hypertensive rats, at 2 and 4 weeks after applying the clip, were anaesthetized, their blood pressures were measured and the kidneys were collected and renal interleukin-6 production estimated. METHOD: The rats were lightly anaesthetized with halothane and prepared for blood pressure measurement via a carotid artery cannula. Interleukin-6 production was estimated from the interleukin-6 messenger RNA (mRNA) present in the kidney tissue. The mRNA species (interleukin-6, beta-actin and renin) were measured by densitometric analysis of the autoradiographs following Northern blot hybridization. RESULTS: The blood pressure was approximately 100 mmHg at all ages in the Wistar rats, but rose from 101 +/- 3 mmHg at age 4 weeks to 154 +/- 2 mmHg at age 9 weeks in the SHR (means +/- SEM, P < 0.01). Constitutive production of renal interleukin-6 could not be detected in either the Wistar rats or the SHR. In 2-K,1C rats the blood pressure was increased significantly at 2 and 4 weeks after clipping to 129 +/- 3 and 140 +/- 4 mmHg (means +/- SEM, both P < 0.01), respectively, and the renal renin mRNA concentration was increased significantly in the clipped and decreased in the non-clipped kidneys at 2 and 4 weeks after clipping. The renal interleukin-6 mRNA could not be measured in either clipped or non-clipped kidneys at either 2 or 4 weeks after clipping. CONCLUSION: These findings demonstrate that renal interleukin-6 was not produced constitutively under normal physiological conditions. Moreover, in spite of the development of hypertension from two causes, genetic and renin-dependent, renal interleukin-6 was not expressed even though there is a deficit in immunological function in the SHR and damage to the renal tissue of the 2-K,1C rats.