While the concept might seem unfair to a thirsty person, it essentially extends to another industry what has become the practice for airlines and other companies that sell products and services to consumers. The falling price of computer chips and the increasing ease of connecting to the Internet has made it practical for companies to pair daily and hourly fluctuations in demand with fluctuations in price -- even if the product is a can of soda that sells for just 75 cents.

The potential for other types of innovations is great. Other modifications under discussion at Coca-Cola, Mr. Baskin said, include adjusting prices based on demand at a specific machine. ''What could you do to boost sales at off-hours?'' he asked. ''You might be able to lower the price. It might be discounted at a vending machine in a building during the evening or when there's less traffic.''

Vending machines have become an increasingly important source of profits for Coca-Cola and its archrival, Pepsico. Over the last three years, the soft-drink giants have watched their earnings erode as they waged a price war in supermarkets. Vending machines have reamined largely untouched by the discounting. Now, Coca-Cola aims to tweak what has been a golden goose to extract even more profits.

''There are a number of initiatives under way in Japan, the United States and in other parts of the world where the technology in vending is rapidly improving, not only from a temperature-scanning capability but also to understand when a machine is out of stock,'' said Andrew J. Conway, a beverage analyst for Morgan Stanley. ''The increase in the rate of technology breakthrough in vending is pretty dramatic.''

Bill Hurley, a spokesman for the National Automatic Merchandising Association in Washington, added: ''You are only limited by your creativity, since electronic components are becoming more and more versatile.''