Antihypertensive treatment has been shown to slow down the decline in glomerular filtration rate (GFR) with time. This has been most extensively studied in patients with diabetic nephropathy and, to some extent, with other forms of renal disease. Angiotensin-converting enzyme (ACE) inhibition has been shown to be more effective in this regard than conventional antihypertensive therapy. This important aspect of antihypertensive treatment has not been studied previously in patients with essential hypertension. Preliminary results regarding the effects of two different antihypertensive therapies on the loss of GFR with time, determined with 51Cr-EDTA clearance after 6, 12, and 24 months of treatment, are presented here. The GFR was assessed in a prospective, randomized, double-blind trial in 257 patients with essential hypertension. All had a normal renal function, and none had diabetes mellitus or glucosuria. The two therapeutic modalities were the ACE inhibitor cilazapril and the beta-adrenoceptor blocking agent atenolol. Both therapies were equally effective in lowering the systolic blood pressure. However, atenolol was slightly but significantly more effective in lowering the diastolic blood pressure after 6, 12, and 24 months. The decline in GFR with time was significantly smaller with cilazapril than with atenolol. After 6 months, the reduction in GFR was 1.0 (cilazapril) vs. 4.0 (atenolol) ml/min x 1.73 m2 (p < 0.01). After 12 months the corresponding changes were 2.0 vs. 4.5 ml/min x 1.73 m2 (p < 0.05) and after 24 months 3.0 vs. 4.0 ml/min x 1.73 m2 (n.s.).(ABSTRACT TRUNCATED AT 250 WORDS)