Effects of monoamine precursors on food intake in male rats were studied. Rats were subjected to an artificial circadian pattern of eating, i.e., a five-hour feeding program during daytime and food deprived during the rest of day. Feeding responses after treatment of drugs were examined. Intraperitoneal injection of L-β-3, 4-dihydroxyphenylalanine (L-DOPA), DL-threo-dihydroxyphenylserine (DOPS), or 5-hydroxy-L-tryptophan (5-HTP) suppressed food intake. The suppression occurred within the first thirty minutes of the test period and their order of potency came out 5-HTP>L-DOPA=DOPS. The suppressive action of 5-HTP was very strong and lasted at least 3 days. The mechanism of this feeding suppressed action by either the central satiety center or the peripheral inhibiting system was discussed in this context.