This paper describes the implementation of WQMS, a general-purpose qualitative modeling environment that integrates the process-centered approach and the constraint-centered approach. A new simulation algorithm is presented in which, owing to this integration, some drawbacks of QSIM are overcome using a depth-first strategy in the assessment of the qualitative states and a reduced set of input variables. With this algorithm it is possible to achieve qualitative simulations in cases in which simulations by QSIM are not feasible, because in some complex systems it is possible neither to specify the initial conditions for all required variables nor to handle the amount of branching that arises with normal computer facilities. An implementation of modeling and simulation of two biological systems is shown; to the best of the authors' knowledge, WQMS is the only environment with which a qualitative simulation can be achieved for these systems.