The effects of thyme on performance parameters in broiler chicken during 35 day feeding period were evaluated. Therefore thyme herb (Thymi Herba, 1,4 % v/w essential oil with 58 % thymol) was given as feed additive to 4 treatment groups (0,1 %, 0,2 %, 0,3 % and 1 %) and tested against a control group. There were no significant differences between the groups in feed intake, daily weight gain, feed conversion and slaughter weight. In addition the main component of the essential oil, thymol, was detected in gut contents, plasma, liver- and muscle tissue by means of HS-SPME (headspace-solid phase microextraction) and GC/MS (gas chromatography/ mass spectrometrie). The increase in thymol levels in the investigated substances correlated with increasing amounts of thyme herb in feed and showed significant differences between the groups. In liver- and muscle tissue thymol levels were under the quantification limit. There was no influence on the antioxidative activity of the plasma, expressed as FRAP (ferric reducing ability of plasma). A molecular approach, PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) was further applied to assess thyme induced alterations in the composition of the caecum microbiota. It seemed that thyme was modulating the composition of the caecalflora. In vitro permeation study through the small intestine using Ussing-Chambers, showed, that the thymol flux seemed to be related to time and concentration.