  We have measured the Cooper Pair Transistor (CPT) in a tunable
electromagnetic environment consisting of four one-dimensional SQUID arrays.
The transport properties of the CPT in the high impedance limit,
Z_env>>R_Q=6.45~k\Omega, are studied for different ratios of the Josephson
coupling energy to the charging energy. As the impedance of the environment is
increased, the current-voltage characteristic (IVC) of the CPT develops a
Coulomb blockade of Cooper pair tunneling and the measured IVCs agree
qualitatively with a theory based on quasicharge dynamics for a CPT. Increasing
the impedance of the environment induces a transition in the modulation of the
IVC with the gate charge from e-periodic to 2e-periodic.
