Chloral hydrate is a widely used hypnotic drug for children and animals but the possible interactions of its sedative action and thyroid hormones has not been investigated. In this study the effect of chloral hydrate on the in-vitro binding of triiodothyronine (T3) to cerebral nuclei of adult rats and on the thyroid hormones' synaptosomal and plasma availability were examined. Our results show that during deep anaesthesia caused by a single intraperitoneal administration of chloral hydrate (100 mg kg(-1)), the maximal number of nuclear thyroid hormone receptors (Bmax) and the equilibrium dissociation constant (Kd) were decreased. These changes returned to normal values when rats woke up (2(1/2)h after chloral hydrate administration). Plasma or synaptosomal levels of thyroid hormones were unaffected during chloral hydrate treatment. Our study demonstrates that the nuclear T3 binding in adult rat brain is affected by the sedative action of chloral hydrate.