Using [3H]arachidonate, ATP, coenzyme A, MgCl2, EGTA and CMP, triacylglycerols were labelled in an isolated neuronal nuclear fraction, N1 (from immature rabbit cerebral cortex). When the radioactive nuclear fraction N1 was subfractionated, 75% of the labelled triacylglycerol product was located in the nuclear envelope fraction, E, indicating that the fatty acid incorporation was taking place at the nuclear membrane. However, when nuclear envelope fraction E was first isolated and then incubated with radioactive fatty acid, a significant incorporation into triacylglycerol was found only when nuclear envelope fractions had been prepared in the presence of dithiothreitol or mercaptoethanol. The use of the thiol compounds during the isolation of nuclear envelope fraction E led to specific incorporation rates (based on phospholipid content) which were at best 45-56% of the corresponding values seen for the parent nuclear N1 fraction. This was seen for nuclear envelope fractions isolated by two different procedures. Specific rates for acyl-CoA synthetase and diacylglycerol generation (by cholinephosphotransferase) were measured in nuclear envelope fractions and found to be similar to specific rates for these enzymes in the nuclear N1 fraction. The deficiency in triacylglycerol labelling in nuclear envelope fractions was likely due to impaired diacylglycerol acyltransferase activity. Higher specific rates of triacylglycerol labelling (82-90% of N1 values) were seen in nuclear envelope fractions assayed very shortly after preparation and in another subfraction of nuclear fraction N1 which contained small amounts of phospholipid and high concentrations of nucleates and protein. These data suggest that triacylglycerol formation may be maintained by the presence of chromatin, while in its absence there is a loss of acylation activity in nuclear envelope fractions.