Abstract: An adapter for interconnecting hollow toy building blocks having a generally cylindrical primary projection extending outwardly therefrom comprises a housing which is generally square in cross section, having two interconnected pairs of generally parallel sidewalls defining first and second open ends for receiving the blocks to be interconnected. The adapter further includes one generally cylindrical tubular member substantially within the center of the housing, the cylindrical tubular member extending generally parallel to the sidewalls and being supported by at least one flange member extending inwardly from one of the sidewalls, the flange member being generally parallel to and of substantially the same height as the sidewalls.