Publisher Summary This chapter illustrates developments in the field of antihypertensive agents. The most significant development has been the Catapresan. This compound is designed and tested initially as a vasocostrictor. Preliminary clinical testing by intranasal administration produced marked prolonged sedation, a lowering of blood pressure, bradycardia, and dryness of the mouth. Two hundred and fifty imidazoline derivatives are synthesized and tested pharmacologically. Contraction of the nictitating membrane in cats, induced by preganglionic electrical stimulation, is not inhibited. No adverse effects on the myocardium are observed in experiments with the heart–lung preparation. No notable change in urine volume and excretion of electrolytes is seen but in renal and essential hypertension, a statistically significant rise of the serum concentration of sodium and chloride ions occurred. No notable change in urine volume and excretion of electrolytes is observed, but in renal and essential hypertension, a statistically significant rise of the serum concentration of sodium and chlrideions occurred. It has been found that the acutely administered drug produced a slowly developing hypertensive effect of long duration in spinal cats but under similar conditions in decerebrate animals, there is no observed effect on blood pressure. The chapter discusses the role of renin in renal hypertension and puts forward a hypothesis containing the novel proposal that the blood pressure lowering capacity of the kidney is directly proportional to its renin content.