The effect of tentoxin at high concentrations was investigated in thylakoids and proteoliposomes containing bacteriorhodopsin and CFoCFp Venturicidin-sensitive ATP hydrolysis, ATP-generated ApH and ATP synthesis were practically 100% inhibited at 2 stM tentoxin, and restored to various extents beyond 50 ttM. With respect to the native enzyme, tentoxin-reactivated ATPase had the following properties: (i) a higher ApH requirement to synthetise ATP; (ii) a decreased futile proton flow through CFoCF~ (without ADP), which remains 100% blocked by ADP. It is concluded that despite its altered kinetic performances, tentoxin-modified CFoCF ~ preserves its mechanism and remains a tightly coupled proton pump.