Human metabolism of the monoterpene cyclic ether 1,8-cineole was investigated in vitro and in vivo. In vitro, the biotransformation of 1,8-cineole was investigated by human liver microsomes and by recombinant cytochrome P450 enzymes coexpressed with human CYP-reductase in Escherichia coli cells. Besides the already described metabolite 2 alpha-hydroxy-1,8-cineole we found another metabolite produced at high rates. The structure was identified by a comparison of its mass spectrum and retention time with the reference compounds as 3 alpha-hydroxy-1,8-cineole. There was a clear correlation between the concentration of the metabolites, incubation time and enzyme content, respectively. CYP3A4/5 antibody significantly inhibited the 2 alpha- and 3 alpha-hydroxylation catalyzed by pooled human liver microsomes. Further kinetic analysis revealed that the Michaelis-Menten K-m and V-max for oxidation of 1,8-cineole in position three were 19 mu M and 64.5 nmol/min/nmol P450 for cytochrome P450 3A4, and 141 mu M and 10.9 nmol/min/nmol P450 for cytochrome P450 3A5, respectively. To our knowledge, this is the first time that 3 alpha-hydroxy-1,8-cineole is described as a human metabolite of 1,8-cineole. We confirmed these in vitro results by the investigation of human urine after the oral administration of cold medication containing 1,8-cineole. In human urine we found by GC-MS analysis the described metabolites, 2 alpha-hydroxy-1,8-cineole and 3 alpha-hydroxy-1,8-cineole. alpha 2005 Elsevier B.V. All rights reserved.