A simple and rapid isocratic LC/MS coupled with electrospray ionization (ESI) method for simultaneous separation and determination of adenine, hypoxanthine, adenosine and cordycepin in Cordyceps sinensis (Cs) and its substitutes was developed. 2-Chloroadenosine was used as internal standard for this assay. The optimum separation for these analytes was achieved using the mixture of water, methanol and formic acid (85:14:1, v/v/v) as a mobile phase and a 2.0 x 150 mm Shimadzu VP-ODS column. Selective ion monitoring (SIM) mode ([M+H]+ at m/z 136, 137, 268, 252 and 302) was used for quantitative analysis of above four active components. The regression equations were liner in the range of 1.4-140.0 microg ml(-1) for adenine, 0.6-117.5 microg ml(-1) for hypoxanthine, 0.5-128.5 microg ml(-1) for adenosine and 0.5-131.5 microg ml(-1) for cordycepin. The limits of quantitation (LOQ) and detection (LOD) were, respectively 1.4 and 0.5 microg ml(-1) for adenine, 0.6 and 0.2 microg ml(-1) for hypoxanthine, 0.5 and 0.1 microg ml(-1) for adenosine and cordycepin. The recoveries of four constituents were from 93.5 to 107.0%. The nucleoside contents of various types of natural Cs and its substitutes were determined and compared with this developed method.