The effects of the inhibitors of biosynthesis and degradation of catecholamines (CA) on photoperiodic induction of flowering were examined in Pharbitis nil , a short-day plant. The inhibitors of CA biosynthesis such as iodotyrosine, methyldopa, methyltyrosine, carbidopa, NSD-1015, and 6-hydroxydopamine inhibited floral initiation when applied before and during a dark period but not after dark treatment, suggesting that these compounds inhibited the production of a flowering hormone in cotyledons. Inhibitors of CA degradation such as SK&F525A, amitriptyline, pargyline, gallic acid, n-butyl gallate, OR-486 and Ro-41-0960 also inhibited flowering, and these inhibitors were effective even when applied after dark treatment, although they did not inhibit floral initiation when applied at 48 h after the end of dark treatment. This result suggested that the inhibitors of CA acted in bud and inhibited the expression of flowering hormone in bud, or the transformation of it into an active substance. The inhibitors used in the present experiment inhibited flowering without the inhibition of vegetative growth. Exogeneous noradrenaline did not induce floral initation under continuous light but partially reversed the inhibitory effect of 6-hydroxydopamine.