The system capacity available to a multi-tier Web based application is often a dynamic quantity. Most static threshold-based overload control mechanisms are best suited to situations where the system's capacity is constant or the bottleneck resource is known. However, with varying capacity, the admission control mechanism needs to adapt dynamically. We propose and implement an adaptive admission control mechanism that adjusts the admitted load to compensate for changes in system capacity. The proposed solution is implemented as a proxy server between clients and front-end Web servers. The proxy monitors dasiablack-boxpsila performance metrics-response time and rate of successfully completed requests (goodput). With these measurements as indicators of system state, we employ a control theory based feedback loop to dynamically determine the rate of admitted requests. The objective is to balance changes in response time and changes in goodput, while preventing overloads due to reduction in available system capacity. We evaluate our mechanism with experiments on a test-bed and find that it is able to maintain higher productivity than a static admission control scheme.