Indicine-N-oxide was analyzed quantitatively in biological samples using a direct partial purification method involving acetonitrite precipitation or methanol precipitation followed by ion exchange chromatography. Trimethylsilyl derivatization of the resultant provided either of two derivatives, depending on the reaction conditions used, both of which had good gas chromatographic qualities. Heliotrine-N-oxide was used as the internal standard for this work. Data are presented to show that this is a reliable and useful internal standard based on its behavior in the partial purification method and on the gas chromatographic characteristics of its two derivatives. In addition, both low and high resolution mass spectral data indicate that heliotrine-N-oxide produces two trimethylsilyl derivatives analogous to those produced by indicine-N-oxide under the same conditions. Application of this procedure to urine and blood samples from cancer patients in clinical trials indicates that over 95% of the drug is removed from the circulation and excreted in the urine over the course of 48 h.