Openings in metal slats too large for it to be labeled a “mouse cage”! I gave it as a gift and unfortunately the mouse could get out through (we believe) an opening in the top. The mouse got out and unfortunately did not survive once we were able to find it. I do not recommend this cage for a mouse. Perhaps a hamster. Even with that being said, the top of the cage has larger openings. Make sure the critter is large enough not to get out.