1. Extraglomerular mesangial cells are strategically positioned between the macula densa and the afferent arteriole. These cells form a syncytium and are connected with glomerular mesangial cells via gap junctions. The model of immunoablation of mesangial cells in anti-Thy-1 glomerulonephritis carries the promise for understanding the function of mesangial cells as potential transmitters of information between the macula densa and the afferent arteriole. 2. The above anatomical relations between structures in the juxtaglomerular apparatus predict several possible routes of information flow. This review charts some hypothetical routes. 3. Research into the messenger systems involved in the transfer of signals from the macula densa to mesangial cells and from mesangial cells to the afferent arteriole suggests several candidate molecules to function in this capacity. Mechanisms of action for each candidate are discussed. 4. The oscillating nature of the afferent signal and efferent function in the tubuloglomerular feedback system, as well as other discoveries, offer a fertile field for future studies.