Brain monoamines are important regulators of affective and cognitive processes and are involved in the aetiology of a number of psychiatric disorders. While methods to probe serotonin and catecholamine function are established, limited methods are available to probe monoamine function as a whole in humans. In the current study, we examined if simultaneous depletion of monoamine precursors can be used as a possible probe of monoamine function. Ten healthy subjects were tested under two treatment conditions; balanced control (B) condition and combined monoamine depletion (CMD) condition. Monoamine precursor depletion was associated with significant reductions in plasma-free tryptophan (46%), tyrosine (74%) and phenylalanine (78%). Greater reductions were achieved for ratios of each precursor to other large neutral amino acids (LNAA); tryptophan/LNAA (86%), tyrosine/LNAA (94%) and phenylalanine/LNAA (94%). Findings suggest that simultaneous depletion of monoamine precursors can achieve significant plasma monoamine depletion in the range expected to affect brain monoamine function.