Penicillin promoted elongation of rice (Oryza sativa L.) seedlings; the relative promotion of shoot elongation was stronger than that of root elongation. Nucleic acids and protein were maintained at much higher levels in seedlings treated with penicillin, the increment of these metabolites being more pronounced in the embryo than in the endosperm. α-Amylase, RNase, and ATPase of endosperm and indoleacetic acid (IAA) oxidase activities of the entire seedlings were enhanced in penicillin treatments. Penicillin caused small increments in respiration rates of seedlings. IAA-induced growth inhibition of intact seedlings could be reversed completely to a growth promotion by joint application of penicillin with IAA. The length of the second leaf sheath of a dwarf rice cultivar was increased by penicillin which further enhanced the gibberellin-induced response. The levels of gibberellinlike substances were increased by penicillin treatments, whereas the IAA level remained unchanged. In penicillin-treated seedlings, more IAA was transported from endosperm to embryo. The stimulating effect of penicillin on rice seedling elongation was partially reversed by abscisic acid and the interaction appeared to be of a noncompetitive nature.