I think both you and Gregory are correct, as it will depend on the value of AutoYield

"Specifies whether an instance of Visual FoxPro processes pending Windows events between execution of each line of user program code."

The question I am not sure about the answer is if the foxpro timer creates a windows event or not. (I always assumed it is, I think it is much safer to think this way and following this assumption I never write code in a timer that uses data in the current data session, I will use a data session object inside the timer if I need to manipulate data)

[UPDATE]
Actually I think you are right Jim, I've been living a lie! code is not interrupted by a timer, I just created a program with a timer and some FOR loops, and if I do not put DOEVENTS inside the loop, the timer never fires, shame on me!