string.Replace(oldStr, newStr)???!!!!

Question

string sqlQuery = "Some huge automated text";
sqlQuery.Replace("S", "");

this "Replace" for some weird reason doesn't work!!! I tried debegging this in "command window". and here's the message from the command window:

? sqlQuery.Replace("as", "bas")
error: 'sqlQuery.Replace' does not exist


I have included "System" in my code -> "using System;". I have tried:
string and String... I'm all out of ideas... what am I missing??

thanx in advance!