The aim of the present study was to investigate the validity of measuring circulatory lactate levels in plasma from Atlantic cod using Lactate Pro^(TM), a portable meter described for measuring whole blood lactate in this species. Atlantic cod were either unstressed or subjected to short- or long-term crowding stress. Whole blood was analyzed on site, and a strong positive relationship was evident between lactate concentrations measured in whole blood and 5 months later in frozen and thawed plasma. Our results indicate that the portable meter can be effectively used also to detect differences in plasma lactate in this species.