The authors examine the issue of robust transaction routing in distributed database systems. A class of dynamic routing strategies which are estimated response times to make routing decisions is studied in details. Since response time estimation and decision making depend on the assumed transaction model and parameers, it is important to examine the robustness or sensitivity to the inaccuracy in the assumptions and parameter values. Through simulations, the authors find that the dynamic routing strategy based strictly on response time is too aggressive in sharing loads and makes too many nonpreferred system routings. It is robust with respect to change in the number of database calls per transaction, but is relatively sensitive to the distribution of database calls. Two refinements are proposed which improve system performance as well as robustness of routing decisions.