American business increased its inventories by one percent in October, mostly in response to rising sales levels, the government reported yesterday.

Commerce Department figures showed inventory levels up $4.23 billion over the month to a seasonally adjusted $421.6 billion.

In September, inventories rose 0.1 percent, or $378 million.

The rise during October didn't appear to have any special significance.

Had sales levels not increased, such a sizable jump in inventories might have raised tears that accumulation was proceeding too rapidly -- possibly worsening any downturn. However, business sales rose 0.9 percent, or $2.66 billion, in October. As a result, the closely watched inventory-to-sales ratio remained unchanged.

The report on inventories yesterday followed a spate of fresh statistics showing the economy still is performing robustly despite forecasts of a recession.

Last week the Labor Department reported the total number of jobs in the economy still was growing rapidly despite 30,000 layoffs in the auto industry last month.

And on Monday the Commerce Department disclosed that retail sales surged 1.8 percent in November, indicating that consumers still haven't begun to retrench.

The series of statistics were expected to intensify the reluctance of the Carter administration to recommend a tax cut in the budget the president will submit in January.

Most of the president's top economic advisers now are leaning against a large antirecession tax reduction, and sources say their position is gaining more supporters.

The figures on inventories showed stockbuilding by retailers rose 1.3 percent in October to $108.97 billion following a 1.4 percent drop the previous month.

Inventories of manufacturers rose 0.9 percent over the month of a seasonally adjusted $223.52 billion. Manufacturers' inventories rose one percent the month before.

Wholesalers' inventories climbed 0.8 percent in October to $89.07 billion after falling 0.4 percent the previous month.

Overall inventory levels nationally were sufficient to cover 1.41 months of sales, unchanged from their September ratio.

The series of recent economic statistics hasn't led economists to alter their forecasts that the nation is headed for a downturn.