select field and goto pick map,in pick map make constrain 
property is true thats called constrain picklist.

HI,

 As usual create 2 picklists among the 1 can be a Parent 
and the other can be a Child now you can configure parent 
child picklists, you can constrain them by doing the steps 
in your Client Application
1.Add the LOV by giving the same LOV_TYPE as "sample"
  for Parent LOVS and Child LOVS the Type should be the 
same as above, after that when you add child values(STATES) 
give the parent Country Value in the PLIC Column.
2. GO to Tools in Tools expand the BC-BC Field_PickMap 
there create 2 pickmaps as usual forthe parent pick map 
make the constrain property to True and in Picklist Field 
property should be "Parent" in parent pickmap

Cheers
Taj