The substance P (SP) analogues [D-Arg1, D-Phe5, D-Trp7,9, Leu11]-SP and [Arg6, D-Trp7,9, MePhe8]-SP (6-11) (antagonists D and G, respectively) are under consideration as new anticancer drugs. In this report, the stability and in vitro metabolism of both antagonists in up to seven different media (water, 1 M acetic acid, human plasma, nude mouse liver and WX 322 human SCLC xenograft homogenized in either 1 M acetic acid or phosphate buffered saline (PBS), pH 7.4) have been characterized by both isocratic and gradient elution reversed-phase HPLC. Antagonist D was stable (never > 13% degradation over 24 h, at 37 degrees C) in water, 1 M acetic acid and plasma but was metabolized by PBS liver homogenates (10%, w/v) sequentially to two stable metabolites with a half life of 0.98 h at a concentration of 500 micrograms ml-1. The major pathway of degradation of antagonist G appeared to be C-terminal methionine oxidation (particularly in plasma) as well as hydrolysis, with even aqueous solutions being significantly affected at low concentrations of peptide (0.1 micrograms ml-1, half life 20.9 h at 37 degrees C). Stable metabolites of antagonist G were also detected in incubations with PBS liver homogenates (half life 1.53 h at 500 micrograms ml-1, 37 degrees C). Overall, the data presented indicate that the modifications made to SP have been relatively successful in preserving chemical and biological stability.