The characteristics of [3H]-prazosin binding in renal cortical membranes of the rat have been assessed under a variety of buffer conditions. At 37° in Krebs' phosphate and Tris buffer, [3H-prazosin bound to two sites, a small population of high affinity sites with properties of α1-adrenoceptors and a much larger population of low affinity sites with different characteristics. High affinity [3H]-prazosin binding was insensitive to Nasu+ Ksu+, Ca2+ and Mg2+ ions, but low affinity [3H]-prazosin binding was markedly increased in Krebs' phosphate or sodium phosphate buffer and further enhanced in membranes pretreated with EGTA. Binding was decreased in the presence of Ca2+, the decrease in binding mainly being due to a decrease in the number of low affinity sites labelled by the ligand. Low affinity 3H]-prazosin binding was increased at 37° and relatively insensitive to α-adrenoceptor antagonists which were weak competitors while catecholamines failed to compete for low affinity binding. Scatchard plots of [3H]-prazosin binding performed using (-)-noradrenaline (1 mM) to define non-specific binding defined binding only to α1-adrenoceptors. This provides a means of differentiating high and low affinity [3H]-prazosin binding.