Oracle Database Discussions

Oracle 8.1.5 (ewww - I know)

I've got a single database on a server called PROD1. In the tnsnames.ora I have entries such as

mydatabase =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = host)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = PROD1)
)
)

PROD1 =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = host)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = PROD1)
)
)

Basically, they're both pointing to the same database. When I set my ORACLE_SID to PROD1 and try to connect, everything connects fine. When I try to use a connect string of username/password@mydatabase, I keep getting an ORA-1034 error - oracle not available. A client trace log file shows trying to use the alias and it appears in the log that everything works fine. A lsnrctl service command keeps incrementing the established connections. Nothing in the listener log regarding a refused connection.

Does anyone know what might be causing this?