The nation's economy remained healthy in August, but the modest gains indicated that the rate of growth is reaching its peak, the National Association of Purchasing Managers said yesterday.

The group's monthly report, based on a survey of 250 purchasing managers at major industrial companies, said new orders grew only slightly in August and production levels were "off a bit after a significant decrease in the rate of growth reported in July."

Robert J. Bretz, chairman of the association's business survey committee, said the economy is "very healthy as it enters a 'plateauing' stage." The latest report "should reassure those who feared the economy was overheating," said Bretz, who is also director of corporate purchasing for Pitney Bowes Inc.

The association said its survey found that most purchasing managers reported slow growth in August for employment, production, new orders, inventories, vendor deliveries and prices. They said these findings were backed up by the Purchasing Managers' Composite Index, which slipped in August to its lowest level since January 1983.

The index consolidates and makes seasonal adjustments to Commerce Department figures on the same five categories covered by the group's survey of members. The index stood at 51.5 percent in August, compared with 56.4 percent in July. A reading above 50 percent indicates the economy is expanding; a reading below 50 percent indicates a declining trend.