This paper studies the determinant of the appropriate rate to fix transmission tolls. I develop a simple model that allows to decompose it in the sum of the risk-free rate and a risk premium. There are two general principles with vhich the rate should conform: (a) it should vary pari passu with the risk-free rate (b) it should compensate the transmission company for: (i) the “risk of regulatory opportunism”; (ii) the nondiversifiable risk imposed to the transmission company by the price-fixing rule; (iii) and also add a liquidity premium. Legislation presently discussed in congress, however, would grant a fixed 10% real rate to the transmission company. Approximating the risk-free rate by the rate paid by a 20-year b ond issued by the Central Bank (PRC) I use the model to estimate the level of risk that justifies a rate of exactly 10%. Finally, I propose a method to regulate transmission fees that would eliminate most revenue risk and would avoid a need to fix a regulated rate.Esta nota estudia los determinantes de la tasa apropiada para fijar peajes de transmisión. Para ello se desarrolla un modelo simple que permite descomponerla en una suma de la tasa libre de riesgo y el premio por riesgo. Los principios generales son dos: (a) la tasa debiera ser variable y variar pari passu con la tasa libre de riesgo: si la tasa libre de riesgo sube, debiera subir la tasa, y lo contrario si baja. (b) Se debería compensar al transmisor por (i) el “riesgo de oportunismo regulatorio”