Ingestion of milk contaminated by radioiodine (most especially I-131) represents potential route of radiation exposure to humans in the event of a nuclear accident. Oral administration of stable iodine to dairy ruminants has previously been suggested as an effective counter-measure to reduce radioiodine levels in milk. However, previous data on its effectiveness are contradictory, and little attention has been given to the most appropriate time to administer stable iodine relative to the time that an animal is first exposed to radioiodine. The effectiveness of 1 g KI orally administered to lactating goats 12h before and 12h after contamination with pulse doses of radioiodine was investigated. Administration of KI prior to the ingestion of radioiodine was shown to reduce the transfer to milk by 50% compared to a reduction of approximately 40% when KI was administered 12h after radioiodine. The administration of KI 12h prior to radioiodine contamination effectively reduced the transfer of radioiodine to the thyroid by approximately 90%. When administered 12h after radioiodine, the KI stopped subsequent uptake of radioiodine by the thyroid. The maximum level of iodine in milk, as a consequence of the administration of KI, was 20 mg I kg(-1). This was 40-fold higher than the maximum permissible level of iodine in milk of 0.5 mg l(-1) currently set in some countries. Further consideration is required on stable iodine administration regimes for different scenarios and the consequent increase in the stable iodine content of the milk of different dairy animals.