If you want to disclaim an account left to you by your spouse, the minimum withdrawal rules state that the final beneficiary for an IRA (or SEP) account can be decided as late as Sept. 30 of the year after the year of the original account owner's death. That means you have until that date to disclaim (give up your legal right) to an account for which your deceased spouse named you the sole beneficiary. Then the account will go to the secondary beneficiary (often your child) or the next heir in line under your spouse's will or state law (if your spouse died without a will).
Why would you ever want to disclaim an account? The typical reason is you don't need the money, and having it would just worsen your estate-tax situation. In this case, it may be better for all concerned tax-wise if the account goes directly to the next person in line. Then that person will have to start taking minimum withdrawals under a separate set of rules for nonspousal inheritors.
Your Spouse's Estate Is the Account Beneficiary
What happens when the designated beneficiary for your deceased spouse's IRA (or SEP) account is his or her estate, you are the estate's sole beneficiary, and you are also the estate's executor or executrix? In this scenario, the old minimum withdrawal rules (which were in effect until 2002) allowed you to roll over the funds in your deceased spouse's account into a new IRA set up in your own name. Then you could follow the minimum withdrawal rules for spousal inheritors.
It's unclear if you can do the same spousal rollover trick under the current rules (although in my judgment, you probably can). If you can't, minimum withdrawals must be calculated under less favorable rules that apply when an account has no designated beneficiary. (Naming the estate as the beneficiary is generally the same as naming no beneficiary for purposes of minimum withdrawal calculations.)