Brennan Linsley/Associated Press

Partygoers smoking marijuana on Tuesday at a bar in Denver during a prohibition-era themed New Year's Eve party to celebrate the start of retail marijuana sales.

DENVER — Colorado embarked on a bold experiment on Wednesday with legalizing marijuana, as shops from downtown Denver to snowy ski resorts began selling the once-illicit drug to any adult with proper identification and a hankering for a hit of Blue Diesel or Kandy Kush.

To supporters, it was a watershed moment in the country's tangled relationship with the ubiquitous recreational drug. They celebrated with speeches and balloons, hailing it as akin to the end of Prohibition, albeit with joints being passed instead of champagne being uncorked.

To skeptics, it marked a grand folly, one they said would lead to higher drug use among teenagers and more impaired drivers on the roads, and would tarnish the image of a state whose official song is John Denver's "Rocky Mountain High." The governor of Colorado and the mayor of Denver both opposed legalization, and stayed away from the smoky celebrations on Wednesday.

While some 20 states allow medical marijuana, voters in Colorado and Washington State decided last year to go one step further, becoming the first in the nation to legalize small amounts of the drug for recreational use and regulate it like alcohol. Ever since, the states have been racing to devise rules detailing how to grow it, sell it, tax it and track it.

In both Colorado and Washington, recreational marijuana has been legal for more than a year. Adults can smoke it in their living rooms, and eat marijuana-laced cookies without fear of arrest. In Colorado, they are even allowed to grow up to six plants at home. But until Wednesday, dispensaries could sell only to customers with a doctor's recommendation and state-issued medical-marijuana card.

Now, any Colorado resident who is 21 can buy up to an ounce of marijuana at one of the 40 dispensaries that began selling to retail customers on Wednesday. Out-of-state visitors can buy a quarter-ounce, but they have to use it within the state. Carrying marijuana across state lines remains illegal, and the plant is not allowed at the Denver International Airport.

Washington's marijuana system is at least several months behind Colorado, meaning that fully stocked retail shelves probably will not be a reality at the consumer level until perhaps June.

While Colorado incorporates the existing medical marijuana system, Washington is starting from scratch, with all of the production and sale of recreational marijuana linked to the new system of licenses, which will not be issued until late February or early March.

"After that, it's up to the industry to get it up and running," said Mikhail Carpenter, a spokesman for the Washington State Liquor Control Board, which regulates the system and is processing almost 5,000 license applications to grow, process or sell.

Growers can start a crop only after they get a license, Mr. Carpenter said, and retailers can sell only marijuana produced in-state by licensed growers when that crop comes in.

With the advent of legal, recreational marijuana, Colorado and Washington have become national petri dishes for drug policy. Their successes or failures will be watched closely by Arizona, Alaska, California, Oregon and other states flirting with the idea of liberalizing their marijuana laws.

Questions still abound. Will drug traffickers take marijuana across state lines, to sell elsewhere? Will recreational marijuana flow from the hands of legal adult consumers to teenagers? Will taxes from pot sales match optimistic predictions of a windfall for state budgets? What will happen to the black market for marijuana?

Skeptical federal authorities are also paying attention. Although marijuana remains illegal under federal law, the Justice Department has given a tentative approval for Colorado and Washington to move ahead with regulating marijuana. But it warned that federal officials could intervene if the state regulations failed to keep the drug away from children, drug cartels or federal property, and out of other states.