check listener trace level.

$ lsnrctl

LSNRCTL> show trc_level
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=xxx.xxxxxxxx.com)(PORT=1521)))
LISTENER parameter "trc_level" set to off
The command completed successfully
LSNRCTL> trace user
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=xxx.xxxxxxx.com)(PORT=1521)))
Opened trace file: /opt/oracle/product/8.1.6/network/trace/listener.trc
The command completed successfully
LSNRCTL> show trc_level
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=xxx.xxxxxxx.com)(PORT=1521)))
LISTENER parameter "trc_level" set to user
The command completed successfully
LSNRCTL> trace off
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=xxx.xxxxxxx.com)(PORT=1521)))
The command completed successfully
LSNRCTL> show trc_level
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=xxx.xxxxxxx.com)(PORT=1521)))
LISTENER parameter "trc_level" set to off
The command completed successfully


if u don't want a lot of log records in listener.log file set:
LSNRCTL> trace off
or
LSNRCTL> trace user