A novel modulation scheme called adaptive threshold level-chaotic on-off keying (ATL-COOK) is proposed. This scheme is applied to direct chaotic communication (DCC) systems where the chaotic signals are used as carrier signals. The objective of the proposed adaptive method is to increase the low BER versus SNR performance caused by the constant threshold voltage level. In the proposed method, the communication signal received by the receiver circuits was defined as a Dirac delta function and a comparison signal was obtained from this signal. Then the BER versus SNR performance was analyzed and compared with that of various chaotic generator structures by using an instantaneous adaptive signal instead of a constant threshold voltage in the receiver circuit decision block. Both the simulation results and the experiments show the efficiency of the proposed method.