The effect of dehydration and sodium depletion on plasma prolactin levels in steer calves is very different from the changes seen in the rat and possibly in man. Removal of drinking water was followed by progressive dehydration for 96 h during which time packed cell volume (PCV) increased from 39.9% to 44.7% and plasma osmolarity (pOsm) rose from 303.3 mOsm to 342.0 mOsm/l with hypernatraemia. At the same time plasma prolactin ( pPRL ) was rapidly reduced from a basal value of 2.3 ng/ml to barely measurable amounts and remained low during dehydration. Restoration of ad lib drinking water was followed by rapid reduction of PCV and pOsm to sub-basal levels during which time the pPRL increased significantly to persist at 15 ng/ml. Sodium depletion was produced by continuous loss of sodium-rich saliva from unilateral fistulation of a parotid duct. During sodium deficiency PCV increased from 38.6% to 45.6% but pOsm fell significantly from 299.9 mOsm/l to 286 mOsm/l with hyponatraemia. As in dehydration, during sodium depletion pPRL was suppressed, and after 7 days was reduced from a basal level of 5.4 ng/ml to 0.5 ng/ml. The sodium depleted steers when given 0.3M NaHCO3, which they consumed readily to restore sodium homeostasis, restored the deficiency gradually in 5 days when pPRL , pOsm and PCV all returned to basal levels without any 'overshoot' or hypersecretion of pPRL . Our finding indicate that extracellular fluid volume changes, not electrolyte content, affect pPRL . This is in agreement with results obtained in the rat, and possibly in man, but the fact that in the steer, the endogenous changes in prolactin level show a profound reduction provides an extreme example of species difference. The means whereby both divergent physiological processes of dehydration and sodium depletion generate stimuli which inhibit prolactin secretion and the relevance of this response in fluid balance homeostasis requires further research.