I have started work on a mod for Morrowind. However, certain items are not appearing in game for me, only in the editor. These are a barrel, and several wall and hallway segments in the Hlaalu tileset. These items appear perfectly well in the CS, but when i enter the game they appear to have vanished into the ether somewhere. Is there any reason for this, and/or any possible fix?

Did you activate the .esp from the launcher?

Yes, and i did basic load order and dependency checks. These items came straight from the vanilla game, and the rest of the ESP, including a custom book, a small room, a dead Nord, and a custom Chargen were all functioning completely correctly. It was just that these specific hallway tiles and the barrel refused to load into the game, and did not appear at all, allowing my to walk into the void at the location where the hallway was intended to go, and not letting me access the items i placed inside the barrel, which included a key to leave the room.

It's hard to say without knowing all the details, but maybe it's a http://www.uesp.net/wiki/Tes3Mod:Mod_Conflicts#Local_References?

Ah, i figured it out. I had placed a rock to cover up a gap in the world, and apparently having this rock there caused everything it touched to disappear. I tested this by moving the rock and messing with what it touched. Weird.

EDIT: I am almost sure that it is some weird issue concerning hallways and rooms leaking into the void, as it is still happening to my door i hope to use as an area transfer.

Were these items added into the world by another ESP or placed by your ESP?