Seventy-two female mice of BALB/c strain were divided into three control and three perchlorate groups. The perchlorate groups drank 1.2% sodium perchlo-rate solution. After 8 or 32 weeks of the 46-week experiment one control and one perchlorate group were totally irradiated with 0.8 Gy on 5 consecutive days. Ten normal thyroid glands, 10 with hypertrophy and hyperplasia, 8 follicular adenomas and 10 follicular carcinomas were histologically and stereo-logically analysed. The results showed significant differences in the average nuclear volume (p < 0.05) and in the average number of the silver-stained nucleolar organiser regions (AgNORs) per nucleus (p < 0.005) between the follicular adenomas and the follicular carcinomas. The method of counting Ag-NORs per nucleus was more valid, more accurate and more economical than the method of measuring the average nuclear volume.