Abstract
A model of the dynamics of intradaily exchange rates is presented. The current Over‐The‐Counter (OTC) exchange rate is the quote of the quoting bank.Two polar cases are considered: (i) If each bank is able to observe the noises relative to the orders of its own clients, then the OTC exchange rate is shown to obey a random walk with a constant conditional variance. (ii) If each bank is not able to observe the noises relative to the orders of its own clients, the OTC exchange rate is no more a random walk and conditional heteroskedasticity appears.