In order to better understand the relationship of free fatty acid (FFA) concentrations to feed intake in laying hens, plasma levels of catecholamine, glucose, and FFA and plasma monoamine oxidase activity were measured. Blood samples were taken in the morning before and after the start of feeding via chronic brachial vein catheters. Plasma concentrations of epinephrine and norepinephrine were analyzed by modified HPLC with an electrochemical detection method. Plasma concentrations of FFA and glucose were not significantly different before and after the start of feeding, however a negative correlation (r = -.763, r2 = .582, P less than .01) between these parameters was observed. No significant correlations between the plasma concentration of FFA and plasma concentrations of epinephrine or norepinephrine before and after the start of feeding were observed. However these were significantly correlated (r = .444, r2 = .197, P less than .05 for FFA and epinephrine; r = .787, r2 = .619, P less than .01 for FFA and norepinephrine) when the behavioral activity of hens was low, such as in resting or feeding. Plasma activity of monoamine oxidase was not different before and after the start of feeding, and no relationship was observed between plasma monoamine oxidase activity and plasma epinephrine or norepinephrine concentration. It is suggested that the increase in plasma FFA concentration before feeding would be elicited by the increase in the circulating epinephrine and norepinephrine concentrations. Plasma FFA concentration was not affected by the increase in circulating epinephrine and norepinephrine concentrations during active behavior such as pacing and egg call before and after oviposition.