Male beagle dogs and miniature swine were given 14C-labelled 8-methoxypsoralen (8-MOP) as a single oral dose (10 mg/kg body weight). In dogs, there appeared to be wide variability in 8-MOP absorption as indicated by the broad range of percentages of radioactivity recovered from urine and faeces over a 4-day period (3.6-24.7% in urine; 47.3-94.9% in faeces); mean recovery values were 13.6% in the urine and 68.6% in the faeces. In pigs, considerable variability in absorption was also evident, but not to the extent of that seen in dogs. Based on the fraction of the dose recovered in the urine, the absorption of 8-MOP was greater in pigs; the proportion of the dose recovered in urine over a 7-day period ranged from 25.8 to 57.8%. Faecal recovery ranged from 15.4 to 49.0% of the dose. Mean recovery values in pigs were 45.4% in urine and 35.6% in faeces. Most of the 8-MOP was cleared from the bodies of dogs and pigs in a few days, and little 8-MOP residue was sequestered in any of the tissues examined in either species. Small amounts of an 8-MOP-related substance remained in the liver and blood for as long as 4 days in dogs and 7 days in pigs.