An editorial writer for the Winnipeg Free Press had a hard time believing Canada was an American ally after the chilly reception he received from the CIA while visiting Washington last month.

Journalist David MacDonald, 41, was one of a handful of reporters from North American newspapers who attended a four-day Washington Journalism Center seminar on America in the 1980s. part of the program included a short, unclassified briefing at CIA headquarters in Langley for everyone . . . except MacDonald. No foreign nationals invited, said the CIA, because someone might spot an undercover agent walking around the compound.

"I called and asked why a Canadian was being treated like an East German," MacDonald says. "I mentioned the fact that we were part of the Olympic and grain boycotts. I didn't mention the Tehran embassy thing. They told me it was just a security rule."

MacDonald also happened to mention the affair to his mother-in-law, Sarah McClendon, the sassy Texas reporter who thinks nothing of lecturing presidents during national press conferences. A CIA director is hardly an awesome figure to McClendon, who picked up the telephone to give CIA chief Stansfield Turner a piece of her mind. Turner missed a McClendon tongue-lashing only because a security man answering Turner's phone hung up on her.

"It was stupid and asinine, and I'm very embarrassed about the whole thing ," says McClendon, referring to the exclusion of her son-in-law. "Canada loves the United States, and we treat it like a dog half the time."

Footnote: A CIA spokeswoman says foreign nationals are not permitted inside the Langley headqauarters because background checks are too difficult to do quickly. But in the case of David MacDonald, intelligence officers might be interested in his next night's activities. Along with McClendon, MacDonald attended a soiree for congressional figures at the White House. McClendon and MacDonald chatted with Attorney General Benjamin Civiletti and ran into Jimmy and Rosalynn Carter. The president insisted the foursome pose for a picture.