The gastrointestinal absorption of neptunium at low mass concentrations as 239Np nitrate (0.5-1 ng Np; 2-5 micrograms l-1) has been found to be 0.03, 0.02 and 0.18 per cent in adult rats, hamsters and rabbits, respectively. Administration as 239Np bicarbonate increased uptake in the rat to 0.15 per cent but had no significant effect on uptake in either the hamster or the rabbit. Absorption in the rat was also increased to 0.14 per cent for 239Np citrate but not for either 239Np phytate (0.04 per cent) or 239Np incorporated into rat liver (0.01 per cent). The fasting of rats for 8 hours increased their subsequent absorption of 239Np as the bicarbonate to 0.25 per cent. The absorption of neptunium at high mass concentrations as 237Np nitrate (0.5 mg Np, 5 g l-1) was increased to 0.26 per cent in the rat compared with the value of 0.03 per cent for 239Np (0.5 ng, 5 micrograms l-1) but a similar effect of concentration was not observed in the hamster. The results support the use of a value of absorption of 0.1 per cent instead of 1 per cent in calculations of annual limits on intake of radioisotopes of neptunium by workers and in estimates of radiation doses resulting from the ingestion of contaminated food and water by adult members of the public.