When your Authorize.net account was first created, you should have received a confirmation email that contained a welcome letter and information regarding your account. It also includes your username and password which you will need to access your Authorize.net Merchant Interface (http://account.authorize.net).

1. Once you have logged in, click on the Account link:
   
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2. Halfway down the page, look for a section called "Security Settings" and click on the "API Login ID and Transaction Key" link.
   
   
   
   

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3. Look for the "Create New Transaction Key" and enter the answer to the secret question you defined when creating your Authorize.net account. Click on the "Disable Old Transaction Key" (only if you know no other provider is using that key) and hit submit. Once you hit submit, your new transaction key will be created.
   
      

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4. Note that if you are looking for your API Login ID, it's located just above the "Create New Transaction Key" section: