The biotransformation of roxithromycin in simulated gastrointestinal fluids at 378C and in rats was investigated by using liquid chromatography-tandem mass spectrometry. Roxithromycin degraded to its Z-isomer and decladinose derivative in simulated gastrointestinal fluids in vitro at pH 3, and followed pseudo first-order degradation with a rate constant (AESD, standard derivation) of 0.1066 min Γ1 (AE0.0014) at pH 1.0, 0.0994 min Γ1 (AE0.0031) at pH 1.2, 0.0400 min Γ1 (AE0.0003) at pH 1.3, 0.0136 min Γ1 (AE0.0008) at pH 1.8, and 0.0022 min Γ1 (AE0.0002) at pH 3.0, respectively. The ratio of Z-roxithromycin to roxithromycin (AESD) was 0.21 (AE0.01) at pH 1.0, 0.19 (AE0.03) at pH 1.2, 0.18 (AE0.01) at pH 1.3, 0.15 (AE0.01) at pH 1.8, and 0.08 (AE0.02) at pH 3.0, respectively. Pepsin and NaCl added to gastric fluid had no effect on the transformation of roxithromycin. Roxithromycin underwent four metabolic routes such as geometric isomerization, demethylation, dealkylation, and hydrolysis of cladinose in rats after oral administration. The geometric isomerization in rats was neither observed after an intravenous dose, nor after an oral dose with Na 2 CO 3 alkalization. The geometric isomerization between roxithromycin and its Z-isomer took place in gastric fluid both in vitro and in vivo. It was interconvertible and pH-dependent. The isomerization of roxithromycin to its Z-isomer was less than that of Z-to E-configuration both in vitro and in vivo.