1-Methyl-1-nitrosourea (MNU) and 1-methyl-3-nitro-1-nitrosoguanidine (MNNG) are carcinogens which methylate nucleic acids and proteins and covalently modify proteins by carbamoylation (MNU) or guanidination (MNNG). Using MNU and MNNG labeled with carbon-14 in the individual carbon positions, the above reactions were quantitated in the isolated guinea pig pancreas, an organ susceptible to tumorigenesis by MNU. Freshly prepared pancreatic lobules were incubated with the labeled drugs (0.03, 0.3 and 1.0 mM) for one hour at 37 degrees C. Alkylated purines from hydrolyzed DNA were separated on Sephadex G10 and acid-soluble nuclear proteins were extracted and separated on polyacrylamide gels. Total uptake of all four labels into lobules was linear with concentration. Acid insoluble radioactivity also increased linearly except for MNNG methylation which plateaued between 0.3 and 1.0 mM. 7-Methylguanine formation by both compounds was approximately ten fold greater than 06-methylation. However, DNA modification by MNU exceeded that by MNNG, especially at the higher drug concentrations. No carbamoylation or guanidination of DNA was detected. Total binding (methylation plus carbamoylation/guanidination) to acid-extractable chromatin proteins was equivalent to DNA modification on a molar basis (approximately 0.35 and 0.08 pmol/microg for exposure to 1.0 mM MNU and MNNG, respectively). All histones were labeled by all drug preparations, with H2A being the principal site of methylation and H2B, H3 and H1 being the major targets of carbamoylation and guanidination. H4 was the least modified histone. Drug binding to cytoplasmic organelles also occurred. These results show a broad spectrum of nuclear and cytoplasmic modification of pancreatic cells by MNU and, to a smaller extent, MNNG.