SA227 pilot reports loosing control of his aircraft as the brakes are released to commence the takeoff roll with full power set. The aircraft turns sharply to the right; but the power is not retarded quickly enough to prevent the aircraft from departing the runway.

Narrative

I received my takeoff clearance from Tower and taxied out onto the runway. The visibility was 3 miles with light rain and the winds were only about 7 KTS. I had just flown in to pick up more freight so while I was taxing onto the runway; I was thinking about my previous takeoff. When I departed; the aircraft pulled to the right; which is not uncommon due to the free castering nose wheel. I lined up on the runway ran through my flow; and decided to hold the brakes longer to try and prevent the plane from pulling right. As I put the power levers up; I glanced down at the torques and they were coming up even; so I let off the brakes to start the roll. As soon as I let go of the brakes; the aircraft immediately took a sharp right turn. This is where I made my mistake because I tried to get the plane straight instead of pulling the power and stopping. It was this three to four second hesitation to stop or continue which I am positive would have made the difference of going off or staying on the runway. I ended up with the nose wheel and the right main stuck in the mud off the right side of the runway; only about 40 FT down the runway from where I started. If this ever happens to me again; I will not hesitate to get the plane stopped first; then figure out how to get it straight.