The binding characteristics of valproic acid (VPA) and its pharmacologically active monounsaturated metabolite, E-delta 2-VPA, to rat plasma proteins were compared. The plasma free fraction was determined by a rapid equilibrium procedure, which minimizes the interfering effects of nonesterified fatty acids liberated by in vitro lipolysis. Nonlinear binding behavior was observed with both compounds over their respective pharmacologic concentration range. Multiple binding-site models were invoked to explain the binding isotherm. The 2-unsaturated compound has a much higher affinity for the rat plasma proteins (mainly albumin) than its saturated precursor. The equilibrium association constants for the high- and intermediate-affinity sites were more than an order of magnitude higher with E-delta 2-VPA than with VPA (10(4)-10(6) versus 10(3) M-1). This difference in binding affinity was also reflected by a lower plasma free fraction for E-delta 2-VPA compared with VPA (much less than 10 versus greater than 20% at total concentrations of less than 100 micrograms/ml). A more pronounced dose- and concentration-dependent variation in the distribution and clearance kinetics is predicted for the 2-unsaturated analogue compared to VPA. Also, the structural dependency in plasma protein binding observed with these branched-chain fatty acids may provide insights into the mechanism of interaction between fatty acyl molecules and albumin.