Carcinogenicities of the N-formyl (N-OH-FAS), N-acetyl (N-OH-AAS) and N-propionyl (N-OH-PAS) derivatives of N-hydroxy-trans-4-aminostilbene (N-OH-AS) were investigated in male and female CD rats. They were injected, i.p. 10 mumol/kg body weight (bwt) twice a week for 6 weeks, and they were killed at the end of 62 weeks. The N-formyl, N-acetyl and N-propionyl derivatives of N-hydroxy-4-aminobiphenyl (N-OH-ABP) were similarly injected at a dose of 100 mumol/kg bwt for comparison in female CD rats. Tumors of the liver, mammary gland and ear duct were produced in the female rats by these N-OH-AS derivatives. N-OH-AAS and N-OH-PAS were more active in the induction of mammary and ear duct tumors than N-OH-FAS. These N-OH-AS derivatives produced more tumors than did the N-OH-ABP derivatives, even at 1/10 dose of the N-OH-ABP derivatives. In male CD rats, these N-OH-AS derivatives produced peritesticular mesothelioma and tumors of the pancreas and ear duct. N-OH-PAS also produced tumors of the small intestine and lung. The acetyl and propionyl derivatives were more carcinogenic than the formyl derivative of N-OH-AS for both male and female CD rats, suggesting that cytosolic acetyltransferases may be more important than the microsomal ones in activating these carcinogens.