1 Valproic acid or phenytoin were added to fresh human serum in varying concentrations and their binding characteristics determined by the method of Scatchard (1949). 2 Changes in serum albumin binding were investigated for phenytoin in the presence of 280, 560, 1050 and 2100 mumol l-1 valproic acid, and for valproic acid in the presence of 40, 120, 280 and 480 mumol l-1 phenytoin. 3 Phenytoin appeared to bind to a single site on the albumin molecule and could be competitively displaced from this site by concentrations of valproic acid above 280 mumol l-1. 4 At high concentrations of valproic acid, the affinity of phenytoin for albumin was greatly decreased but the number of available binding sites was increased from one to four. 5 Valproic acid was bound to two high affinity and five low affinity binding sites but the latter were not detectable at valproic acid concentrations below 2100 mumol l-1. 6 Phenytoin displaced valproic acid from its high affinity binding sites, although this was statistically significant only at a concentration of 480 mumol l-1 phenytoin.