If you paste the same block of time repeatedly, one block following the other, you can create a looping segment in your animation. To create a smooth looping animation, you need to exclude either the first or last key of the copied block to prevent keys from doubling up at the ends.
The key to be excluded must be at the exact start time of the copied block of time.
To animate a loop by copying and pasting keys:
At least one track in the block should start with a keyframe. Also, for a smooth loop, the first and last frame of the selection should be the same.
 Extras: Dope Sheet. The Exclude Left End Point and Exclude Right End Point buttons appear.
The selection is copied to the clipboard, minus the first frame.
The last position key should be your insert point.
3ds Max opens the Paste Time dialog.
3ds Max pastes the time, including keys.