Hypothalamic magnocellular neurons constitute a good model of neurochemical plasticity, because a single neuron can express various combinations of neuropeptides and enzymes under different physiological conditions. Tyrosine hydroxylase has been shown to occur ectopically in various non-catecholaminergic neurons. We investigated the expression of tyrosine hydroxylase and its possible role in the magnocellular neurons of the supraoptic and paraventricular nuclei in salt-loaded and lactating rats, using in situ hybridization and immunohistochemistry, alone or combined, in light and electron microscopy. Our results demonstrated that almost 25% of the magnocellular neurons in the supraoptic nucleus and 15% in the paraventricular nucleus expressed tyrosine hydroxylase in salt-loaded rats, and 10% in the supraoptic nucleus of two-day lactating rats. Double labelling showed that this tyrosine hydroxylase was essentially synthesized in magnocellular neurons expressing vasopressin. The ultrastructural localization of tyrosine hydroxylase was less homogeneous in the cytoplasm of magnocellular neurons than in periventricular neurons. In lactating and salt-loaded rats, magnocellular neurons were devoid of the catecholamine biosynthesis markers aromatic L-amino acid decarboxylase, L-3,4 dihydroxyphenylalanine, dopamine and GTP-cyclohydrolase I. Tyrosine hydroxylase expression did not increase after rats were injected with reserpine. Our results indicate that the phenotype of the magnocellular neurons expressing tyrosine hydroxylase in lactating and salt-loaded rats is non-catecholaminergic, and suggest that this tyrosine hydroxylase might be involved in osmoregulation.