Toxicity data of 82 compounds, tested in both a sub-acute and a sub-chronic study, were evaluated to find out whether it is possible to substitute a sub-acute for a sub-chronic study without losing quantitative information. The no-effect level (NEL) and the minimal-effect level (MEL) were used as the basis for comparison. For each of the compounds, the sub-acute and sub-chronic studies were performed under similar conditions, with the same strain of rats and with a comparable range of dietary levels. It appeared that the NEL in the sub-acute study was equal to the NEL in the sub-chronic study for 56% of the compounds. For 44% of the compounds reviewed, the NEL in the sub-chronic test was lower than the NEL in the sub-acute test, indicating that a sub-chronic study cannot simply be replaced by a sub-acute study. However, it may be acceptable to use the NEL obtained in a sub-acute study as a basis for establishing acceptable daily intakes or permissible exposure levels for humans if an additional safety factor of 10 is applied. Furthermore, it was concluded that the conventional sub-acute study, including some selected haematological and biochemical parameters, seems extremely useful to obtain quick and reliable information on the toxicity of the numerous non-regulated chemicals which have to be investigated in the years to come.