Gerard Butler is a Potty Mouth -- Literally!

Gerard Butler takes a leak in the on-set porta-potty during a film break in Pasadena, Calif. on Monday afternoon.

Gerry, 38, is filming The Ugly Truth with Grey’s Anatomy actress Katherine Heigl, who was seen squatting and smoking as a chimney (as usual).

The Ugly Truth is about a romantically challenged morning show producer (Heigl) reluctantly embroiled in a series of outrageous tests by her chauvinistic correspondent (Butler) to prove his theories on relationships and help her find love. His clever ploys, however, lead to an unexpected result.