On April 28, Donald Kendall set aside the rigors of outsmarting the competition and moved to Washington to become president of the Chamber of Commerce of the United States.

But the competition took a shot on the chin as soon as he got here.

Kendall, it seems, used to be chairman of Pepsico, bottler of Pepsi-Cola. And the soda machine in the junk-food room at the Chamber's 1615 H St. NW. headquarters contained (gasp!) Coca-Cola.

But only briefly. Ver-r-r-ry briefly.

"The decision was made internally--and shortly after he arrived--that it would be a nice thing to do to change," said Milton Mitler, head of media relations for the Chamber.

"We had had Coca-Cola in there for quite a while," Mitler pointed out.

So presto, change-o: for the past few months, the once offending drink machine has dispensed only Pepsi, Diet Pepsi, Teem and Orange Patio. All are manufactured by Guess Who.

Mitler promises, however, that a new machine is on order and may be in place this week. It will contain both Coke and Pepsi.

Do the Coke people like the new soft drink lineup? Says Mitler: "They have not complained."