One method for assessing mesozooplankton production is based on measurements of the activity of an enzyme, aspartate transcarbamylase (ATC), in samples of the whole mesozooplankton community. The first held experiments were carried out in neritic areas, the south-western part of the English Channel and the west of Brittany. Previously published results have shown two main characteristics of ATC activity variations, i.e. an allometric relationship with biomass and a close correlation with the mesoscale changes with time of the mesozooplankton biomass. Other experiments were performed in two oceanic areas, the Ligurian Sea and the upwelling system off the southern coast of Portugal. They illustrated strongly different patterns of ATC activity variations. In the Ligurian Sea, it has not been possible to establish any allometric relationship, probably owing to either fairly complex hydrodynamics or disturbance of the mesozooplankton system caused by an unusual biotic environment. Off Portugal, however, allometric relationships clearly characterized two subsystems, one quite close to the shore and the other in offshore stratified oceanic waters. These results give an overview of different patterns of ATC activity variations in natural systems: some early hypotheses are confirmed, limits in applicability of the method are highlighted and promising prospects are becoming clearer. (C) 1995 International Council for the Exploration of the Sea