BY TRANSPLANTING genes into the egg cells of ordinary mice, scientists have succeeded in producing mice of twice the normal size. It took a team of scientists from four major institutions to do the job which, we are told, will open new vistas of gigantism.

The scientists see much immediate payoff from their new technique for accelerating animal growth. Research on genetic diseases will be advanced. Beefier cattle and porkier pigs may some day populate the barnyard. No doubt the game of basketball will be in for another major transformation.

We don't want to seem ungrateful for this singular breakthrough, but we wonder if the scientists thought this thing all the way through. Scientific progress has a funny way of sneaking up on us. Before anyone has thought much about the consequences, mankind finds itself in possession of technological marvels -- artificial organs, super-accurate missiles, test-tube babies -- that open up vast questions about the society's ethical values and distribution of resources.

What are the unforeseen consequences of proliferation of the new technology? How can it be rationed and controlled? How much money should be devoted to it when so many ordinary needs are still unmet?

Our immediate concerns about this latest advance are more mundane. We had thought that whatever need existed for a super-rodent had already been amply filled by the traditional rat. We note that at least some of the giant mice are able to transmit their enlarged state to their progeny. No doubt subsequent generations will be carefully watched, at least for awhile. But mice are adept at multiplying, and we suspect that in time some of them will escape the laboratory. Wouldn't it have been a better idea to start off by trying to build, as they say, "a better mousetrap" -- or at least a bigger housecat?