One way to do this would be to use a COMMON block within your function that you wish to integrate with QROMB. For example if you had this function: function cubic, x common share, a, b return, x^3 + (x - a)^2 + b end you could define this common block in your main program as well, and then when you update "a" and "b" in your loop, the change would take place in your "cubic" function as well. Check out the IDL Help docs on COMMON blocks and on the QROMB function for more information, there are some good examples there. -Josh ITTVIS