Cheat codes:
     Enter these at the cheats screen:

Unlock all cheats:
     L1,  L2,  R2, R1, L1, L2, R2, R1, L1, L2, R2, R1, L1, L2, R2, R1,
Circle,  Circle,  Circle,  Circle, X, X, X, X, Square, Square, Square,
Square, Select, Select, Select, Select.

Level select:
     Circle, X, Square, Square, X, Square, Circle.

Infinite ammunition:
     L2, Circle, R2, Square, Circle, L2, R1.

All weapons:
     R2, X, L1, Square, R1, Circle, L2.

Double damage:
     Square, Square, Square, Circle, Circle, Circle, X.

Invincibility:
     L1, Square, Circle, Circle, Square, L1, L2.

Temporary invincibility:
     L1, L1, L1, L1, L1, L1, R2.

Invisibility:
     Square, X, Circle, Square, X, Circle, Square.

Full ego:
     R1, R1, Circle, Circle, L1, L1, R2.

Full armor:
     L1, L1, R1, R1, X, X, Circle, Circle.

First person view:
     L2, R1, L1, R2, Circle, X, Square.

Big head Duke:
     Square, Square, X, Circle, Circle, X, Square.

Small head Duke:
     Square, X, Circle, Circle, X, Square, Square.

Big head enemies:
     X, X, R1, X, L1, X.

Small head enemies:
     X, L1, X, R1, X, X.

Wacky FMVs:
     L1, L2, R1, R2, Circle, Circle, Square, Square.

Outtake FMVs:
     L1, L2, R1, R2, Square, Square, Circle, Circle.

Ending FMV:
     Circle, R2, L1, Square, L2, X, R2.