How to deploy Windows Service with different identities in different environments?

I need to deploy a Windows Service to several environments and it supposed to run under different user identities in different environments. I can create environment dependent variables to hold custom user name and password, BUT in some environments the service supposed to run under Local System account. What do I assign to those variables for the Local System?

Thank you!
Konstantin

It looks like NT AUTHORITY\SYSTEM works! Problem solved.