It will also start taking orders for the larger model, but the units won't ship until next month.
The model with the 21.5-inch (54.6-centimetre) screen will cost $1,299 and up, depending on the configuration.
The model with a 27-inch (68.5-centimetre) screen will start at $1,799.
The iMac tacks the computer components to the back of a large LCD screen. The new models have no disk drive, helping make the edges one-fifth the thickness of the old model. They bulge considerably toward the middle of the back, however.
Apple revealed the new models a month ago.