Although environmentalists don't have much to laugh about these days, Orlo, a Portland, Ore.-based environmental education group, wants to help lighten the mood. Its free exhibition of environmental cartoons called "The Last Laugh" is now showing at The Art Gym on Marylhurst College campus until May 20, featuring more than 150 editorial cartoons from all over the country. They send up all ideological stances, from saving endangered species to saving jobs for loggers, and the cartoonists even take a few jabs at pretentious greens. The show is sponsored by Orlo and the Art Department of Marylhurst College with a grant from the National Endowment for the Arts. Orlo, which uses the creative arts to educate people about environmental issues, can be reached at Box 10342, Portland, OR 97210 (503/242-2330).