Using Ajax, how to fetch one cck field value to another cck field( to get value in php code)

Hi,

Using Ajax, How to fetch one cck field value to another cck field( to get value in php code). Otherwise is there any option to retreive the first cck field value into php code of second cck field so that , to get related data of first one. Both are select list only.

Pls help me,

Thanks in Advance,
Suji