Male and female inbred CBA mice were treated with different concentrations of p-fluorophenylalanine (pFPA) for 4 weeks and their subsequent longevity was recorded. Young males treated with 4 × 10-4M pFPA had their average longevity reduced by 123 days in comparison to controls, but a five-fold higher concentration had less effect. 1-year-old females treated with 4 × 10-4 M pFPA had their subsequent average longevity reduced by 77 days, and again a higher concentration had no significant effect. The longevity of 3- to 4-week-old females and 1-year-old males was also unaffected by the analogue. The control and treated mice were marked by ear punch and mixed in cages, so environmental effects could be largely discounted. The results indicate that the incorporation of an amino acid analogue into protein can reduce longevity, as predicted by the protein error theory of ageing. It is also clear that there is a complex interrelationship between the analogue dose, the age and sex of treated animals, which will require further stud