Pittsfiel is the largest city in and the county seat of Berkshire County, Massachusetts, United States. It is the principal city of the Pittsfield, Massachusetts Metropolitan Statistical Area which encompasses all of Berkshire County. Its area code is 413. Its Zip Code is 01201 (01202 and 01203 are Zip Codes for Pittsfield P.O. Boxes). The population was 45,793 at the 2000 census and remains relatively stable. It is the county seat of Berkshire County, and it continues to be one of the population centers of Western Massachusetts, although the population has declined in recent decades.