We propose a synthesis for test approach in which multiplexers are embedded in the behavioral models of the various modules constituting a hierarchical system. This approach can also be applied to system-on-a-chip designs in which synthesizable models are available for the embedded cores. The embedded multiplexers provide complete, single-cycle transparency, thereby offering a straightforward yet effective solution to the problems of test data propagation and test vector translation. In order to determine I/O bitwidths for single-cycle transparency, a global analysis is carried out using a graph-theoretic framework and an optimization method based on integer linear programming. Case studies using high-level synthesis benchmarks and an industrial-strength benchmark show that synthesis for transparency introduces very little area and performance overhead.