A distributed computing system, such as a Grid, could be a very dynamic environment and the user groups are most likely become highly diverse. A user group could be formed by the users of different networks, organizations, or administrative-domains with different hardware/software infrastructures and managerial policies. Handling requests from a wide range of users from different domains becomes a challenge when attempting to accommodate all the differences. Service providers find it impossible to track all users (the number of users could be potentially very large) in a Grid. Therefore, an access control mechanism that provides users appropriate access to the resources in a dynamic environment is required. Role Based Access Control (RBAC) models have been demonstrated to be an effective and efficient approach for an administrator to manage accesses in a computing system. Much has been done to adapt the RBAC concept to Grids and focus on the authorization and verification of the dynamic factors or contexts of a user, such as time, location, rank, etc. Some applications also allow administrators to change the policies during the authorization process, but they did not handle the authorization in real-time and on-demand manner in a Grid. It is a critical authorization requirement for a dynamic environment. Therefore, this problem motivated us to develop a new dynamic authorization protocol, Dynamic Role Lease Authorization (DRLA) that is suitable for a dynamic distributed computing environment.