The computer mentions that portals leak high energy gamma radiation. She also says that you are an excellent test subject. Lies?
Head left from the raised platform and some steps will appear. Go up them. Once you're at the upper level, the steps will once again descend, leaving a big pit. To the left, there is a weighted storage cube on a ledge that you can't seem to be able to reach. What you will have to do is create a portal in that pit area, then make a portal on the wall that faces the weighted cube, to the left of the window into what looks to be an empty observation chamber, the likes of which you have seen in other test chambers. Make the wall portal at the very corner, pointing at the cube. Then fall into the floor portal in the pit and you should have enough momentum to land on that platform. If you don't make it all the way to the level with the cube on it, you should be able to jump (press SPACE or whatever you set jumping to) to get to the cube. Take the cube and drop to the floor with it.
Now those stairs will reappear, so go down them and put the cube onto the red button. The chamber lock will open, so go through. There is poison water on the floor, so be careful here. You don't necessarily need to use those platforms, because you can just shoot a portal into the hallway beyond the portals and make another one to walk through But be careful not to walk into the high energy pellet.
Once you're in the room with the pellet, make a portal on the wall that it hits of the same color as the portal that you just walked through (otherwise, when the pellet goes through, it will hit you). Now, you can adjust the position of the other portal such that you can walk through behind the high energy pellet without having to worry that it will ricochet soon and hit you in the face. Walk through and then get out of the way to make sure the pellet can't hit you. Then, make sure you remember which color of portal the pellet comes out of, because you need to make a portal of that color above the containment thing near the red button. Once you have done that correctly, the platform down the stairs will descend so that you can get to the elevator to the next chamber.