The effect of hypothyroxinaemia on postnatal progression of the motor nerve conduction velocity was studied in 33 very low birthweight infants. Serum concentrations of thyroid stimulating hormone, triiodothyronine, and thyroxine were determined at birth and at ages 3, 7, and 21 days. Nerve conduction velocity was measured in the first week of life, on day 21, and at 40 weeks' postmenstrual age. Seven infants maintained their thyroxine concentration above 60 nmol/l (4.67 micrograms/100 ml) throughout the study. Three of these infants needed mechanical ventilation and one had an intraventricular haemorrhage. Twenty six infants developed hypothyroxinaemia (thyroxine less than 60 nmol/l). The nerve conduction velocity was delayed in 13 infants, two on day 21 and 11 at 40 weeks' postmenstrual age. The delay was equivalent to 1.9-4.4 weeks. All these infants belonged to the group with depressed thyroxine concentrations. The delay in progression in nerve conduction velocity was associated with prolonged hypothyroxinaemia, especially in those infants who also required ventilation. Further studies are in progress to study the effect of thyroid hormone on the nerve conduction velocity in preterm infants.