In vivo injection of the edible frog Rana esculenta with NOS inhibitor, L-NMMA caused prolongation of skin allograft and xenograft viability, statistically significant only in the latter case. In the present studies skin allo- and xenografts at the latent or rejection phase were excised from the hosts (Bufo bufo, R. temporaria, and R. esculenta) and incubated in vitro for 24 hrs in a medium only or in the presence of competitive (L-NMMA, L-NAME, L-aminoguanidyne) and noncompetitive (dexamethasone and cycloheximide) inhibitors of NO synthesis. In some experiments graft infiltrating cells were washed out and cultured separately from the respective skin fragments. The nitrite level was measured in the culture supernatant using Griess reagent. The nitrite level was negligible in the control skins, autografts, and xenografts depleted of graft infiltrating cells, as well as in allo- and xenografts excised at the rejection phase. In the case of grafts excised at the latent phase, the nitrite amount was substantial in supernatant from allografts and significantly higher in xenografts. A high level of nitrite was also present in supernatants from graft infiltrating cells. It is concluded that the NO contributes to some stages of the rejection process of the anuran skin grafts, this contribution being especially significant in the case of xenografts. The main source of this agent are graft infiltrating phagocytic cells.