Experiments are described which have resulted in the development of more efficient EMS-treatment method for seeds. The method drastically reduces the amount of EMS-induced physiological damage and sterility, without changing the mutation frequency. It also allows the storage of EMS-treated seeds for a virtually unlimited period of time, and it improves the reproducibility of effects. Barley seeds were treated with EMS and subsequently subjected to one or several of the following sequential post treatments: ( a) post washing for 0–36 hr, ( b) redrying and ( c) storage of the redried seeds at +24 and −20 °C. EMS-treatment was conducted for 3 hr at 24 °C in each case. Two concentrations were used, the higher one (3·6%) reduced the seedling height by 60 per cent, compared to the control. Flowing tap water at 4 l/min was used at +24 °C for post washing. Redrying was performed at 40 °C in a vacuum (30 torr) over P 2O 5. With increasing post washing time seedling height, survival rate and fertility generally are increased. The frequency of chlorophyll mutations remains unchanged. Redrying of EMS-treated seeds which were post washed for only a short period of time or not at all, results in 100 per cent killing. A sufficiently long period of post washing (24 hr) allows the redrying of the seeds without increasing either physiological damage or sterility (as compared to the same period of post washing without redrying), while the mutation rate remains constant. Storage at 24 °C for 14 days of seeds which were treated with 3·6% EMS, post washed and redried leads to complete lethality. However, using cool conditions (−20 °C) after a sufficiently long period of post washing (24 hr), the redried seeds can be stored for a virtually unlimited period without increasing either physiological damage or sterility; the mutation rate also remains the same. It is concluded that the EMS-concentration in the tissue is decreased by extraction and dilution during post washing, while the sensitivity to the mutagen increases at the same time. It is assumed that under our experimental conditions the two opposite processes are in equilibrium; as a consequence, the mutation rate remains unchanged. Part of the physiological damage and sterility appears to be caused not by EMS itself but by its hydrolysis products. Since post washing reduces physiological damage and sterility but has no effect on the mutation rate, it may be concluded that more (toxic) hydrolysis products than active EMS are removed from the seeds. The results of storage at +24 °C suggest that even with long post washing treatments (36 hr) a small residue of EMS and its secondary products still remains in the tissue; storage for 14 days has a similar effect as a treatment for 14 days with a low EMS concentration; all the seeds die. Storage at a sufficiently low temperature appears to inhibit all EMS-activity and the originally induced EMS-effects are preserved without change.