Differential pulse polarographic assay of intact nitrosoureas revealed the lower bioavailability of CCNU (1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea) in stomach and liver after oral administration to rats in comparison to BCNU (1,3-bis-(2-chloroethyl)-1-nitrosourea); blood levels of CCNU were below the detection limit of the method (20 ng). After i.v. bolus the CCNU concentration in plasma fell faster than that of BCNU. The rate of CCNU decomposition during incubation with blood at 37 degrees C was 3 times lower than that of BCNU.