Low level intake of drugs from the ingestion of contaminated feed may lead to residue problems in food animals. Sulfadimidine (SDD) was used as a model to determine the residue risk at various doses in laying hens. The drug was administered as a single intravenous injection (100 mg kg(-1) body weight, BW), as a single oral dose (100, 30, 10, 3, 1 mg kg(-1) BW) and via medicated feed for 7 consecutive days (30, 10, 3 mg kg(-1) BW). Drug levels were determined with HPLC-UV for plasma, yolk and albumen. Pharmacokinetic values, which were calculated using a first-order one-compartment model, residue levels and transfer rates into the eggs were found to be dose-dependent. Even low doses of 3 and 1 mg kg(-1) BW resulted in measurable residues in yolk and albumen 1 day after a single oral administration. After ingestion of medicated feed at 3 mg kg(-1) BW, mean drug levels at 0.14 +/- 0.01 microg g(-1) were found in albumen and at 0.09 +/- 0.01 microg ml(-1) in plasma. Generally, the residue levels in albumen and plasma were higher than in yolk. These findings demonstrate a residue risk for the consumer even after low level intake of drugs.