This paper proposes a Multivariate-Arch in Mean model to analyze the potential channels through which domestic fiscal and monetary policy as well as changes in the international economic environment may affect interest rate differentials across countries. This technique is illustrated by analyzing the behavior of short-term interest rates in a number of European countries prior to the introduction of the common currency. The key feature of our results is that macroeconomic variables exert both a direct and indirect influence on the shortterm interest rate differential. This indirect effect is captured through the conditional volatility of the differential, which is itself a statistically significant determinant of the level of the differential. This relationship is likely to be overlooked by more traditional models that focus solely on the first order moments of the process.