The genotoxic air pollutant 1-nitropyrene (NP) labelled with 14C was administered to rats as the pure compound (6 micromole) by intraperitoneal injection, and vapor-phase-coated onto diesel particles (380 ppm 5 mg/rat) by oral and intratracheal instillation (i.t.). In all three cases well over 50% of the 14C label was recovered within 24 H, 20-30% in the urine and 40-60% in the faeces. Following i.t., most of the particles were recovered in the lung, trachea and oesophageal cavities; after oral dosing particles were mainly in the faeces. The similarity in elimination kinetics and metabolite profile following such diverse doses and routes of administration indicates that NP is readily released from diesel particles both in the lung and in the gastrointestinal tract, and that the dose range used (10 microgram to 10 mg/kg body weight) is either well above or well below the level needed to saturate metabolism and excretory mechanisms in the rat. Accumulations of 14C remained in the lung and gastro-intestinal tract 24 h after i.t.; these organs might therefore be particularly vulnerable to possible damage by NP.