The uptake of³ H-indole-3-butyric acid (IBA) in Arabidopsis thalianaseedlings grown in liquid culture was studied and compared with other natural and synthetic auxins. It was shown that the uptake of IBA and also of indole-3-acetic acid (IAA) was linear during 1 h, but it was saturable after 5 and 12 h, respectively. The K_m value for the saturable in vivo transport of IBA was 0.18 μmol/L, compared with 0.19 μmol/L for IAA, 3.1 μmol/L for naphthylacetic acid (NAA) and 1.2 μmol/L for phenylacetic acid (PAA). The uptake of ³ H-IBA was strongly inhibited by unlabelled IAA. The K; value for IAA was determined as 1.7 μmol/L. Long distance transport of IAA and IBA (acropetal and basipetal) has been investigated using 10 cm stem segments of Arabidopsis. In general, both hormones were transported better acropetally than basipetally. IBA was taken up better than IAA and was also transported more rapidly, although for both hormones a transport further than 6 cm after 4.5 h in the stem could not be detected. Our results indicate no prominent differences between IAA and IBA transport in Arabidopsis thaliana.