A PVG rat strain with total deficiency of C6 and partial deficiency of C2 (PVG/c⁻), and a syngeneic control strain (PVG/c⁺), were used to study the production of extrahepatically synthesized complement. Livers of complement deficient rats were transplanted in sufficient rats (Tx−L). The C6 and C2 levels in Tx−L rats declined within 2 days to 25% and 30%, respectively, and remained stable for more than 6 weeks. To investigate the contribution of C6 synthesis by the liver, C6 sufficient livers were grafted in deficient rats (Tx+l). After an initial increase, with maximum C6 levels of 119% at 10 days following transplantation, the C6 levels decreased gradually and C6 was no longer detectable 28 days after transplantation. This decline in C6 levels was dependent on antibody production against C6. No significant change in the C3, C4, factor H and factor B levels was observed. Expression of C6 mRNA in the grafted PVG/c⁺ sufficient liver was comparable to the expression of C6 mRNA in control PVG/c⁺ livers while C6 mRNA expression in the transplanted PVG/c⁻ liver and the control PVG/c⁻ liver was lower. In conclusion, it was demonstrated in vivo that not only C6 but also C2 is synthesized extrahepatically in PVG/c rats.