To obtain more information on a possible involvement of prolactin during parr-smolt transformation of Atlantic salmon, Salmo salar, parr and smolt populations of the same age were sampled from February to June. A clear separation of the two populations was observed according to their size and gill (Na+ K+)-ATPase activity. Comparison of plasma prolactin (PRL) levels between these two populations indicated a significant decrease from May to early July in smolts whereas, in parr, a steady increase was observed. Diel variations in plasma PRL levels at three different dates were also studied in fish undergoing smolting. Sampling fish over a 24-h period on 11 March, 5 May and 15 July did not indicate the presence of a diel rhythm in PRL release. These data support the hypothesis that the PRL decrease is associated with smolting in Atlantic salmon and that such a decrease cannot be explained by a change in diel rhythm of plasma PRL levels.