The growth of the gross national product will slow next year, but the state of the economy is still a question mark, according to a survey of nearly 500 corporation presidents around the country.

Industry Week Magazine released the results of its sixth annual business forecast yesterday at a press conference here.

On the positive side, according to the survey, an increase of sales and manpower requirements are predicted bt the chief executives.

But the survey also revealed a belief that there will be a higher rate of inflation in 1978.

At the press conference, Eaton Corp. president Paul Miller, Pioneer-Standord Electronics, Inc. President Preston Heller and North American Bitruder Co., Inc., president Robert Jennings critized the regulatory climate of the Carter administration as a major cause of business uncertainty about the economy.

"The regulatory climate is irregular," said Heller. "I can't think of one example of reduction of regulation since the Carter administration took over - something they talked about considerably during the campaign."

Heller cited a statement by Federal Communications Commission as an example of the government doing more arm than good.

"They announced that the would approve 40-channel Citizen Band radios," Heller said. "And then, in an offhand remark, they said that consumers shouldn't but 23-channel units anymore because they are obsolate. Well, they aren't obsolete. Two of the channels have 95 per cent of all traffic."

"But," Heller added, "that offhand remark almost destroyed an industry."

Miller said the regulatory climate "continues to get worse with the airbags and skid control regulation as prime examples."

"The government is too active in setting regulations without knowledge. They don't really understand the industries they are regulating," he said.

Other survey results:

More than 55 per cent of the executives see the current business pace as better than last year. But in last year's survey, 72 per cent said they felt this year would be better.

Only 40 per cent forsee better business conditions six months from now, while 45 per cent say things will be better in a year.

Sales are expected to rise 9.4 per cent in the manufacturing sector and 8 per cent for non-manufacturers.

Almost half those responding indicated that their firms plan more capital spending next year than this year.

Some 42 per cent of the chief executives see a need for increasing their number of employees, a figure that gives rise to a optimistic prediction on the unemployment situation by Industry Week Economics Editor Dale Sommer, who supervised the survey.