Yes and no. Lassie never took a dump during filming. In fact, a simple procedure was used to insure that she wouldn't do so. Before filming began, one of Lassie's trainers would insert a cork in her anus, where it remained until filming was completed. Afterwards, Jon Provost, the kid who played Timmy, was forced to go into Lassie's pen and remove the cork. That may have been a factor in Provost's decision to forego an adult acting career and become a real estate salesman.