Yes, you can. But really, really try to avoid it. See for example: http://matlab.wikia.com/wiki/FAQ, or search for discussions on the matlab newsgroup. In short globals make code tricky to debug because there is only one global namespace - meaning if you happen to have 2 groups of functions that happens to use the same variable name there will be one variable that will be set by functions from both groups _and_then_used == errors will be caused by functions that are completely unrelated to where it blows up.
A presonal note: I included rather rapidly a few globals in a number of functions in a toolbox I wrote ~15 years ago - it was a poor design choise, but I still haven't completely weeded them out.
So sit back, take a second glance at your problem and draw up a good clean design. I've heard people use globals for very large matrices, but then others have suggested class-definitions and other tricks, if this is your problem you could also look at memmapfile.
