When cucumber seeds were sown on agar containing L-3, 4-dihydroxyphenylalanine (L-DOPA), the growth of cucumber seedlings was strongly inhibited by L-DOPA immediately after sowing. However, they recovered from the inhibitory effect of L-DOPA 12 days after sowing. Throughout the experimental period, blacking of agar was observed in the medium containing L-DOPA due to chemical alteration of L-DOPA in agar associated with a polymerization reaction. In the roots of cucumber seedlings treated with L-DOPA, accumulation of L-DOPA and free amino acids was observed compared with the control. Especially, the amount of tyrosine and phenylalanine with a similar chemical structure to that of L-DOPA increased remarkably.<br>It was assumed that the increase was due to the metabolim of L-DOPA to other harmless amino acids. This transformation of L-DOPA could be one of the detoxification mechanisms responsible for the recovery from the inhibitory effects of L-DOPA.