CHICAGO (AP) _ A record 9.2 inches of rain soaked the Chicago area Friday, flooding homes and expressways, isolating O'Hare International Airport and knocking out power and telephone service to thousands of people.

At least two people died during the storm, which hit hardest in northern Cook County, which encompasses Chicago, and Du Page County to the northwest. Some 300 people fled their homes, and at least one hospital, in suburban Des Plaines, had to evacuate patients.

Gov. James R. Thompson declared the two counties disaster areas, authorizing state aid for residents.

The governor also activated 200 National Guard troops to provide security and aid in evacuation efforts.

The storm overtaxed the Metropolitan Sanitary District's 31-mile network of pipes known as the Deep Tunnel, filling the 1-billion-gallon capacity Main Stream Tunnel and sending more than 1 1/2 million gallons of raw sewage and rain water into Lake Michigan, said district spokeswoman Barbara Rumsey.

''All of those local sewers go in to our sewers and all of it has been backed up all night and all day,'' she said.

Flooding closed the Kennedy Expressway leading out of Chicago to O'Hare, making it ''virtually impossible'' to reach the airport from the nation's third-largest city, said Marilyn Katz of the city's Department of Aviation.

American Airlines spokesman Steve McGregor said the airport was ''an island just surrounded by water.''

He said as many as 75 percent of American Airlines' flights would be canceled; United Airlines also suspended most operations until 8 p.m.

''A Herculean effort is being maintained by hundreds of people at O'Hare to keep it open, and it is open,'' said Federal Aviation Association spokesman Mort Edelstein.

One runway was open for landings, and two partial runways were open for takeoffs, he said, adding, ''City crews and FAA personnel are working doggedly to get those runways cleared of water and water away from navigational aids.''

Even the weather service couldn't escape the storm. ''We're stranded,'' said meterologist Bob Collins, noting 3 feet of water surrounded the National Weather Service building near the airport and 6 inches of water had crept inside.

More rain was forecast for the weekend.

Collins blamed the storm on an unstable air mass that socked in the Chicago area, caused by cool air to the north and warm, very moist air to the south.

The same system dumped heavy rains on parts of Iowa, Kansas, Missouri, Wisconsin and Michigan, where another traffic death was reported. Brunswick, Mo., received 5.8 inches of rain, while Hudsonville, Mich., got more than 4.4 inches.

The 9.2 inches of rain received by mid-afternoon at O'Hare washed away the previous 24-hour record of 6.24 inches set July 12-13, 1957, the National Weather Service reported.

At least 300 cars were stranded as water levels hit 6 feet on some expressways in and around Chicago.

A 35-year-old truck driver was thrown through his windshield and killed when his vehicle went out of control and crashed after hitting a flooded area on the Kennedy Expressway.

In northern Illinois, a 23-year-old woman died when her car skidded off a rain-soaked highway near Ashton into a culvert holding 8 feet of water, said state police Sgt. Steven Earhart.

Police used boats to rescue some motorists from the tops of their cars, said Sgt. Jack Kimsey of the department in northwest suburban Wheeling.

''It's very bad out here. It's backing up into our sewer, backing up into our toilet. It's just flushing out of our toilet,'' said Mary Machak, 63, of Mount Prospect.

Three feet of water filled the Machaks' basement.

''We're up to our knees,'' she said.

''When they were walking in the street it was up to their waist-level,'' said neighbor Helen Sorger. ''We had like like a little rapids in our driveway, like a waterfall.''

The storm knocked out phone service for several hours to 30,000 Illinois Bell customers in suburban Chicago, said utility spokesman Doug McFarlan. Flooding, wind and lightning also cut electricity to an estimated 21,000 Commonwealth Edison customers in the region, said spokeswoman Diane Kania.

Mike E. Cramer, a spokesman for the Chicago Transit Authority, said shuttle buses were dispatched to take riders over 6 1/2 miles of the Chicago Loop- O'Hare rapid transit line because of flooding on the tracks.