We have found that etiolated pea (Pisum sativum L. cv. Alaska) seedlings grown under true and simulated microgravity conditions on a 3-D clinostat showed automorphosis and automorphosis-like growth and development, respectively. We have also suggested an important role of auxin polar transport for automorohosis in etiolated pea seedlings since auxin polar transport activity of etiolated pea seedlings were substantially inhibited under true and simulated microgravity conditions. This suggestion was confirmed by the fact that automorphosis was phenocopied by the application of auxin polar transport inhibitors. In Arabidopsis auxin polar transport has considered to be regulate by efflux and influx carrier proteins located in plasma membranes, AtPINs and AtAUX1,respectively. In order to know how gravistimuli affect auxin polar transport in etiolated pea seedlings at molecular levels, strenuous efforts has been done, resulting in successful identification of PsPIN2 and PsAUX1 whose products are considered to locate in plasma membrane and to be involved in auxin transport. Significantly high levels in homology were found on deduced amino acid sequences among PsPIN2,PsPIN1 and AtPINs, and also found between PsAUX1 and AtAUX1. The expression of PsPIN1 and PsAUX1 were suppressed under simulated microgravity conditions. On the other hand, the expression of PsPIN2 was affected little under simulated microgravity conditions. These results suggest that PsPIN1 and PsAUX1 are genes encoding putative efflux and influx carrier proteins for auxin polar transport, respectively although the function of PsPIN2 has not been clear yet. Based on the results described above, a possible role of PsPINs and PsAUX1 for auxin polar transport in etiolated pea seedlings will be discussed.