A specific radioimmunoassay (RIA) for measurement of red drum (Sciaenops ocellatus) somatolactin (rdSL) was developed using somatolactin purified from red drum pituitaries and antisera to rdSL raised in rabbits. The rdSL antisera specifically bound to cells in the pars intermedia bordering the neurohypophysis in red drum and Atlantic croaker (Micropogonias undulatus) pituitaries. These cells correspond to the periodic acid-Schiff-positive cells and are distinct from the lead-hematoxylin-positive cells which include the alpha-melanocyte-stimulating hormone (alpha-MSH) cells. No cross-reaction in the rdSL RIA was observed with red drum growth hormone (GH), prolactin (PRL), recombinant tuna GH, Atlantic croaker maturational GTH (GTH II), human GH, PRL, human follicle-stimulating hormone alpha subunit (FSH alpha), bovine FSH beta subunit, human adrenocorticotropic hormone, synthetic mammalian alpha-MSH, beta-MSH, gamma-MSH, delta-MSH, or cortisol. Dilution curves of plasma and pituitary extracts from red drum and Atlantic croaker were parallel to the rdSL standard curve. The assay had a sensitivity of 0.14 ng/ml plasma when 100 microliters plasma was assayed. SL levels in juvenile red drum ranged from 0.5 to greater than 32 ng/ml in plasma and from 0.5 to 13.5 micrograms/mg tissue in pituitaries. No significant elevation in plasma SL levels was observed in either red drum or Atlantic croaker exposed to acute stressors. In contrast, plasma SL levels were significantly higher in both red drum and Atlantic croaker exposed to dark backgrounds than in those exposed to light backgrounds. These results provide the first evidence that SL may play a role in adaptation to dark backgrounds in sciaenid fishes.