We describe the use of a model system to mimic chronic toxin exposure, similar to that which might be found in a human situation, where exposure to dietary or environmental toxins occurs at a low level for an extended period of time. This is in contrast to the acute, immediately toxic dose effect usually observed in flask tissue culture. The apparatus used was a flow cell bioreactor in which cells can be cultured for lengthy periods of time as a continuous viable population. The compound used as the toxic agent was 3-nitropropanoic acid (3-NPA), a fungal neurotoxin which acts as a suicide inhibitor of succinate dehydrogenase. The secondary human cell line, Int 407, was tested in conventional flask culture for periods of up to 72 hours to determine the no-observed-effect level (NOEL) for 3-NPA. Cell populations established in the bioreactor were continuously exposed to levels of 3-NPA below that of the NOEL for periods of up to 4 weeks and the viability of the population determined using MTT, trypan blue and ATP assays.