The standard view of the monetary transmission mechanism rests on the central bank's ability to manipulate the overnight interest rate by controlling reserve supply. In the 1990s, there was a significant decline in the level of reserve balances in the US accompanied at first by an increase in federal funds rate volatility. However, following this initial rise, volatility declined. In this paper, we find evidence of structural breaks in volatility. We estimate a Tobit model of temporary open market operations and conclude that there have been changes in the Desk's reaction function that played a major role in controlling volatility.