A grazing experiment was carried out using 18 Holstein/Friesian dairy cows at the Scottish Agricultural College, Crichton Royal Farm in 1991, to investigate the use of n-alkanes for estimating supplementary grass silage intake by grazing dairy cows. Two groups of animals grazed perennial ryegrass swards and were offered a supplement, consisting of a perennial ryegrass silage, twice daily after milking. Mean group silage intakes were measured daily during the last 5 days of a 12-day experimental period and individual silage intakes were estimated using n-alkanes. Animals were dosed twice daily with paper pellets containing dotriacontane (C32). The silage was marked with hexatriacontane (C36). The mean silage intake, estimated by weighing, was 6·8 kg DM per day. The mean estimated silage intakes were 6·9, 8·7 and 8·3 kg DM per day respectively using odd-chain n-alkanes in the C27−C35 range of naturally occurring alkanes, the odd-chain n-alkanes in the C27−C35 range with C36, and C36 by itself. No significant differences were found due to sampling routine (morning or afternoon) when using the naturally occurring n-alkanes but significant differences did exist when using the C36 alkane in the calculation of supplement intake. The results indicate that the n-alkane technique can be used to estimate the supplementary silage intake of grazing dairy cows using naturally occurring n-alkane patterns.