A gas chromatographic method for the determination of cysteamine and its disulphide cystamine is described. Cysteamine and cystamine are converted into N,S-diisobutoxycarbonyl and N,N-diisobutoxycarbonyl derivatives, respectively. The derivatives are analysed by gas chromatography with flame photometric detection, using a DB-210 capillary column. The calibration curves for cysteamine and cystamine in the range of 0.2–5.0 nmol are linear and sufficiently reproducible for quantitative analysis, and the detection limit is about 0.5 pmol injected. Cysteamine in mouse tissues is found in the free reduced, free oxidized and protein-bound forms. Free oxidized and protein-bound forms are reduced to free cysteamine by the use of sodium borohydride, and then derivatized. Cysteamine and cystamine in mouse tissues can be measured without any interference from coexisting substances by this method. The recoveries of cysteamine and cystamine added to the tissue samples are 91–106%, and their reproducibilities are found to be satisfactory. Analytical results for the determination of various forms of cysteamine in mouse tissues are presented.