Abstract: A field-grown green Chinese onion plant is used for investigation. Local treatment of the exposed vein with acetylcholine (ACh, the nerve transmitter), cytochalasin B (CB, the specific inhibitor of microfilament) and amiprophos-methyl (APM，the specific inhibitor of microtubule) are carried out in pursuit of the possibility that protoplasm in the “sieve tube-companion cell complex” is actively engaged in phloem transport. Their effects on transport are estimated by content of N, P and K in the leaf blade. Inhibition effects of both CB and APM increase steadily with their concentration, whereas ACh behaves differently, exhibiting promotion in range of lower concentration. All the results support the postulation that the peripheral protoplasm in the sieve tube serves not only as a passive semi-permeable membrane, but also directly involved in phloem transport.