While Google has been installing adverts alongside Gmail messages for years, these new ads appear as messages that can be opened like e-mails and forwarded to others.
The adverts appear in the new "promotions" tab of Gmail's new multi-tab interface, and they're marked with a pale yellow background and labelled "ad" just in case you can't spot them.
In a statement, Google said that the ads are part of the Promotions tab in the new inbox in Gmail. You can see the old style of advert if you disable the Promotions tab.
The adverts are likely to hack off those who came to Gmail because it is so good at weeding out spam. Now it seems that Google has purged inboxes from spam to peddle spam of its own.
Google said that the advertising will keep Google and Gmail free to use. It said that it worked hard to make ads safe, unobtrusive, and relevant.
Google also said the new ads are more relevant than earlier Gmail ads. They replace the old-style ads above the inbox or to the right of messages unless people disable the Promotions tab.
Apparently the adverts can't be marked as spam, but if you close them they will go away - until you refresh the browser.
Users can go to Gmail's ads preferences manager to block specific advertisers.