General Motors Corp. and Ford Motor Co. saw their share prices plunge on Thursday as concerns grew that the struggling auto makers may not have enough cash to survive the deepening downturn in the U.S. economy.

General Motors closed down 31%, or $2.15, at $4.76, and Ford fell 22%, or 58 cents, to $2.08, in New York Stock Exchange composite trading. GM, which hit an intraday low of $2.26, its lowest point since 1951, ended the day with a market value of $3.9 billion to Ford's $6 billion.

The...