Numerous studies have shown during the past 10 years that adenosine is present in the normoxic kidney and accumulates when ATP hydrolysis prevails over ATP synthesis. Local generation of adenosine by the macula densa cells and its release into the interstitium of the juxtaglomerular apparatus (JGA) is considered to be the link between the enhanced NaCl concentration in the tubular fluid and the subsequent responses including preglomerular vasoconstriction and reduction of renin release by the juxtaglomerular cells. Micropuncture and microperfusion experiments using specific adenosine agonists and antagonists support the concept that adenosine functions as a mediator in the signal transmission of the JGA.