glDrawPixels()?

I am displaying RGB images in stereo using OpenGL. I send to glDrawPixels() an array of size (3 * XDim * YDim) where each word is 1 byte and every three sequential bytes contains an R, G, and B value for a pixel. When displayed the image looks correct as far as color, but it is repeated three times horizontally. When I send RGBA information to glDrawPixels() everything again looks correct, i.e. the information was used and reflected in the display, but then I get four images horizontally. Not a coincidence I suspect, but why is this happening? Anybody know? Hints? Thanks.