In two groups of eight patients, concentrations of thiopentone in mature breast milk and colostrum following anaesthesia induction with 5.4 and 5.0 mg kg-1 b.w. (mean), respectively, were measured in the first 36 h postoperatively. Blood concentrations were measured simultaneously. The maximal concentrations were: in mature breast milk, 3.4 +/- 0.68 mumol l-1 (mean +/- s.e. mean) (0.090 mg 100 ml-1), and in colostrum, 1.3 +/- 0.5 mumol l-1 (0.034 mg 100 ml-1). The milk/plasma ratio was less than 1.0 in both groups. The above concentrations may be regarded as negligible and therefore non-toxic for the nursing infant.