It sounds like you might need to use pop-up menu instead of the drop down list.
First a picky detail: records cannot appear in fields. A record consists of many fields. But you can select a value from a value list and records in a portal matching that value can appear. Data in fields can be looked up (copied) from fields in other records and data can be simply displayed from another record.
Much depends on what you want to do and how you design your layouts. All of these approaches require using Manage | database | Relationships to first create a relationship of some sort.
Here's a link that may help you get started: Auto Fill
Google "conditional drop down filemaker"
Google Filemaker site "conditional drop down site:filemaker.com"
Whoops - always read the post twice...