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The retardation of growth and maturation of the thyroidectomized foetal lamb suggests that maternal thyroid hormone does not cross the ovine placenta in sufficient quantities to permit normal foetal development (Hopkins & Thorburn, 1970). Assessments of the permeability of the ewe's placenta to thyroxine (T₄) have previously been undertaken using labelled hormone. Dussault & Fisher (1970) found no evidence of placental transfer, although other workers have concluded that transfer does occur (Comline, Nathanielsz & Silver, 1970; Burrow, Anderson & Quilligan, 1969). Long-term studies investigating the transport of unlabelled T₄ have not been reported. The aim of the present study was to investigate the transfer of maternal T₄ across the placenta of the ewe during the last 6 weeks of pregnancy.

Surgical thyroidectomy was performed on two foetal lambs of approximately 110 days' gestation. Indwelling carotid catheters (1·5 mm o.d., 1·0 mm i.d.) inserted at the time of