1. The kinetics of sulphadiazine were studied in well‐nourished and under‐nourished subjects. 2. The metabolic clearance rate of a single dose of sulphadiazine given either orally or intravenously was faster in under‐nourished subjects. 3. The plasma protein binding of the drug was found to be reduced in under‐nourished subjects. 4. Blood concentrations (48 h and 72 h) of sulphadiazine tended to be low in the under‐nourished as compared with well‐nourished subjects after administration of multiple doses of the drug. 5. It is concluded that a revision in the dosage schedule may not be necessary in the treatment of under‐nourished subjects with sulphadiazine, since the blood concentrations were higher than the reporte d minimum to combat infection.