Many inherited diseases occur in pure-bred dogs, but diagnosis at the level of DNA is impossible because the canine genome is largely unknown. Random amplification of polymorphic DNA (RAPD) provides many polymorphisms, but the reproducibility and Mendelian inheritance are not beyond doubt. An optimized polymerase chain reaction (PCR) was developed for canine DNA with respect to the annealing temperature and the concentrations of MgCl2, template DNA and primers. RAPD amplification products were in the range of 100-1500 base pairs. With six primers, 21 different reactions with different electrophoretic patterns were obtained, yielding 9-29 products per reaction. In DNA from dogs of 16 different breeds, 14% of the products were polymorphic; when only beagles were included the rate of polymorphism was 10%. All of the reaction products were completely reproducible in 16 DNA samples. Mendelian transmission was analysed in six beagle families (42 dogs). The segregation of polymorphic amplification products in 21 reactions performed on DNA from all beagles was nearly complete; in only two of the 630 reactions was there a product that could not be traced back to either of the parents. The reproducibility and Mendelian behaviour of polymorphisms generated by RAPD in dogs makes this tool very suitable for development of DNA markers of canine inherited diseases.