Summary The incidence and morphology of thyroid neoplasms in different strains and stocks of rats are reported. The frequency of thyroid tumours varied to a great extent between different strains of rats. Low incidences were observed in DA/Han (0·85 per cent) and BDII/Han rats (1·25 per cent), while somewhat higher incidences occurred in Han:WIST rats (7·3 per cent). The highest number of thyroid tumour-bearing rats was found in the Han:SPRD stock (55·5 per cent). Thyroid neoplasms were classified histologically as polymorphofollicular adenomas, papillary adenomas, cystadenomas, polymorphofollicular carcinomas, papillary carcinomas, C-cell adenomas and C-cell carcinomas (medullary carcinomas). C-cell neoplasms were predominant in Han:SPRD rats (93·0 per cent), while the majority of tumour-bearing Han:WIST rats (61.4 per cent) had follicular tumours.