The possibility that placentally-transferred labetalol might cause sympathetic blockade in the newborn infant was examined by measuring systolic blood pressure, heart and respiratory rates, palmar sweating, blood glucose and the metabolic and vasomotor responses to cold stress. Measurements were made serially over the first 72 h of life in 22 term infants, 11 born to mothers treated with labetalol and 11 carefully matched controls. Infants of mothers treated with labetalol showed a mild transient hypotension which had disappeared within 24 h [mean systolic blood pressure at 2 h, 58.8 (SEM 2.4) mmHg compared with 63.3 (SEM 3.0) mmHg for controls, P less than 0.05]. There were no other significant differences between the two groups. It is concluded that labetalol does not cause clinically important sympathetic blockade in the mature newborn infant.