The conversion of ochratoxin C to ochratoxin A was studied in rats after oral and intravenous administration. The concentration of ochratoxin A in the blood as a function of time was the same after oral administration of equivalent amounts of either ochratoxin C or ochratoxin A. The maximum ochratoxin A concentrations were measured 60 min after administration. Given intravenously, ochratoxin C was also converted to ochratoxin A. Maximum concentrations were reached after 90 min. It is concluded that ochratoxin C is readily converted to ochratoxin A after both oral and intravenous administration. There is reason to believe that a comparable toxicity of the two toxins is based upon this conversion and that only interference with the biotransformation mechanisms may cause a difference in their toxicity.