When microsomes prepared from aged potato tuber slices, were incubated for 10 min with [ 14C]oleoyl-CoA, in the absence of NADH, three labelled molecular species of phosphatidylcholine (PC) were mainly formed: 1-palmitoyl-2-[ 14C]oleoyIPC (50.1%), 1-linoleoyl-2-[ 14C]oleoyIPC (31%) and 1-[ 14C]oleoyl-2-linoleoyIPC (10%). Three other molecular species were less heavily labelled: 1-[ 14C]oleoyl-2-palmitoyIPC (4.9%), 1-linolenoyl-2-[ 14C]oleoyIPC (2.4%) and 1-[ 14C]oleoyl-2-linolenoylPC (1.6%). Acylation of oleoyl residues occurred preferentially in position 2 of sn-glycerol. When NADH was present in the incubation medium the following labelled species ( 18:3 18:1− PC , 18:2 18:1−PC and 16:0 18:1−PC ) accumulated for the first 10 min of incubation. These species were then progressively desaturated by the oleate desaturase of microsomes and clear precursor-product relationships could be observed between 18:2 18:1−PC and 18:2 18:2−PC on one side, and 16:0 18:1− PC and 16:0 18:2−PC on the other. The following molecular species accumulated as final desaturation products in microsomes: 1-palmitoyl-2-[ 14C]linoleoylPC (38.8%), 1-linoleoyl-2[ 14C]linoleoylPC (36.8%), 1-[ 14C]Iinoleoyl-2-linoleoylPC (12.3%), 1-[ 14C]linoleoyl-2-palmitoylPC (6.8%), 1-linolenoyl-2-[ 14C]linoleoylPC (3.5%) and 1-[ 14C]linoleoyl-2-linolenoylPC (1.8%). No selection among oleoylPC molecular species was realized by potato oleate desaturase.