A method of operating a first device 100 in a wireless communication system is proposed. The method may comprise: generating a MAC PDU related to a groupcast transmission; and selecting at least one SL resource for a transmission of the MAC PDU among a plurality of candidate resources in a resource pool, wherein a first SL resource included in the at least one SL resource is included in a first SL DRX active time, and the first SL resource being selected outside the first SL DRX active time is not allowed, based on the MAC PDU being related to groupcast transmission.