The effects of clonidine on renal hemodynamics and renal function make it a particularly useful antihypertensive agent. During treatment of hypertensive patients with clonidine, renal blood flow and glomerular filtration rate are well maintained, and renin secretion is reduced. Early in therapy, a slight tendency to retain salt and water may be seen as blood pressure is lowered. This effect on salt and water excretion is usually transient and may be avoided if a diuretic is used concomitantly. No deterioration of renal function was noted in patients with primary hypertension who were treated with clonidine for periods from 6 months to at least 5 years. The drug is effective in patients with renal hypertension with or without renal failure and well tolerated. Clonidine is also effective in hypertensive patients undergoing chronic hemodialysis, but doses may have to be reduced because the drug is excreted chiefly by the kidney.