Toad Editor and SQLPlus Execute command

One of my co-workers recently upgraded from 10.5 to 10.6. This week he started to use a script that he uses for one of our tasks. The script is executed step-wise in order to observe the results. After upgrading, some of the commands no longer worked. they were of the form:

execute procedure;

We tried F9, highlight and execute button, cursor on and execute button. All failed with ORA-06550, “encountered symbol not expected.” Highlighting and F5 (run as script) worked. He finally changed to exec instead of execute and it worked. I have a several script files that I have used by copying commands to SQLPlus for years that use exec. As far as I can tell execute is a valid SQLPlus command.

Could someone explain this situation?

TIA,
Dave Reed