The administration of nicotine during the perinatal stages of life resulted in a significant decrease in tumours occurring after transplacental induction by N-methylnitrosourea (MNU). The overall tumour incidence following p.o. application of MNU to dams was 85% in rats of the F1-generation, the main occurrence being related to the neurogenic system (62% of the animals). Regular injections of nicotine before or after birth resulted in a reduction of malignancies by 17% and 22% (P = 0.08 and 0.0015), respectively. The difference in the incidence of neurogenic tumours proved to be highly significant (P less than 0.002) in rats of either sex, when nicotine was applied over 26 weeks following birth. There was a gender-specific imbalance in rats which received the carcinogen only, in favour of a lower tumour yield in females (P less than 0.04), which became less apparent when nicotine was given additionally. These findings suggest that nicotine is capable of modulating the expression of chemically induced tumours of the neurogenic system in a favourable way.