North Sea oil drilling operations employ oil-based muds which contain aromatic compounds likely to be genotoxins. Recent legislation has controlled use and discharge of the muds to try to minimise their effects. In an attempt to include a procedure in the ongoing monitoring programme in the North Sea, which could detect genotoxic effects, the sister chromatid exchange (SCE) technique has been developed in the plaice, <i>Pleuronectes platessa</i>. Preliminary work on the plaice involved developing the techniques for cytological examination and for obtaining sister chromatid differentiation. A standard technique using kidney tissue was adopted and is described and compared with that of other studies. SCE levels were measured after intra peritoneal (ip) injections of various substances in order to test the technique. Mitomycin C (MMC) commonly used as a positive control in genotoxicity studies, elicited a significant increase in SCEs when administered at 10^-4mg/g fish and resulted in mitotic inhibition at a dosage of 10^-3mg/g fish. Diesel oil and low toxicity base oil (used in drilling muds) were injected at 1.0ml/100g fish and the fish killed after periods of time. Diesel produced a peak in SCE level, almost significantly higher than control levels 7 days after injection. The low tox oil produced a significantly increased level of SCEs at 5 days after injection. Results are discussed in context of other similar studies. Environmental conditions were simulated in an experiment by exposing fish to sediments mixed with diesel and low tox oil based drilling cuttings to give sediments with theoretical oil concentrations of 5 and 50 ppm. Results show a slight but insignificant increase in SCE levels in fish exposed to sediments containing 50 ppm diesel. In a subsequent experiment fish were exposed to diesel based and different low tox oil based drilling muds in sediments, at theoretical oil concentrations of 30 ppm. No difference in SCE levels was found between any of the groups. Suggestions are put forward to explain these inconclusive results and problems encountered with the technique are discussed. A field sampling programme was carried out where plaice were collected from areas close to drilling installations in the North Sea and from control areas. SCE levels in both groups of fish were low and not significantly different. Sediment sample analysis showed that the control area was contaminated with oil, thus fish were probably exposed to higher levels of oil than in experiments. Results are discussed and an assessment of the use of the technique at sea was made. Results were analysed using analysis of variance (AOV) on transformed data, and by high SCE frequency analysis (HFC) (Carrano & Moore, 1982) in order to compare the sensitivity of the two methods. There was little difference in the sensitivity of the methods, and HFC analysis appears to have no advantage over the more established AOV. It was concluded that although the SCE technique was successfully developed in the plaice, several limitations were recognised and further studies are required before the assay could be used routinely in monitoring studies.