The expression and fate of exogenous DNA (lacZ gene), introduced into the gonads of chimaeric embryos and chickens that had been produced by transfer of primordial germ cells (PGCs) transfected in vitro, were examined. PGCs obtained from embryonic blood were transfected in vitro by lipofection and transferred to the partially sterilized recipient embryos. Expression of the lacZ gene was observed in the gonads of chimaeric embryos incubated for 3 days after the PGC injection (71.2%, 37/52). Introduction of the lacZ gene into the gonads of chimaeric embryos was confirmed by PCR analysis. The percentage of embryos with gonads positive for the lacZ gene was 95% (38/40) after 3 days of incubation after the PGC injection. The lacZ gene, however, appeared to persist episomally but was gradually lost during embryonic development. After 17 days of incubation after the PGC injection, the lacZ gene was detected in only 14.3% (3/21) of the embryos examined. Although the lacZ gene was detected in the gonads of two hatched chicks (11.1%), it was not detected in the gonads of chimaeric chickens at sexual maturity. Offspring derived from the lipofected PGCs were obtained from the chimaeric chickens at frequencies of 12.1-69.9% in males and 71.6-97.6% in females. The technique developed in the present work could be used to test the expression of exogenous DNA in the gonads of early chicken embryos and should facilitate the production of transgenic chickens.