PIX and multiple subnets on the inside

I recently purchased a PIX 501 and am stuck.

I have three internal subnets, two of them can be reached through a router on the first subnet.

I installed the PIX between the first subnet and the Internet. All of my subnets can reach the internet.

I’m not able to reach one subnet from another with the PIX set as my gateway. This is a problem. Has anyone else run into this?