Pls specify clearly what type of messages u have to send between the dialogboxes...If u want to acess the variables in other dialogboxes just make an object of that dialogbox and then call the variable..
dont forget to add the header of the calling dialogbox..
If both dialogs are in the same application (process) then WM_COPYDATA is not needed. If both dialogs are in the same application and both use MFC, then you just need a pointer to the dialog and a user message id.
Please specify what you need help with. Just saying "how to send messages between two dialogs" does not ask what you really need help with. It is vague and makes us guess at what you need.