In two experiments, in the growing season March–May, freshly harvested double‐low oilseed rape (Brassica napus) plants were fedad libitumto penned roe deer (Capreolus capreolus) and red deer (Cervus elaphus). In Experiment 1, oilseed rape was fed to five roe and 12 red deer as 60% of their daily dry matter (DM) intake for four weeks, after a two‐week dietary change‐over period. The rest of the diet comprised cattle‐rearing pellets (33%) and freshly cut heather (Calluna vulgaris) plants (7%). In Experiment 2, oilseed rape was fed to seven roe and eight red deer as 100% of the diet for up to six weeks, after a two‐week change‐over period.In both experiments, Heinz bodies were found in roe deer blood 2–3 weeks after the start of the change‐over period, with a fall in mean packed cell volume (PCV) and blood glutathione (GSH) concentration. In Experiment 2, after 5–8 days of feeding on 100% oilseed rape, four roe deer showed signs of inappetance and were changed to a diet of 80% oilseed rape. At the same time, the other three roe deer had very low PCV (<60% of normal values) and oilseed rape feeding was discontinued for those animals. The red deer showed no symptoms of haemolytic damage nor any change in blood GSH concentration in either experiment, but showed a gradual fall in PCV in Experiment 2. No other ill‐effects were observed in any of the animals.Intakes of oilseed rape (gDM/kg0–75/day) were lower for the roe deer than the red deer in both experiments. The severity of haemolytic anaemia in the roe deer was related to the proportion of oilseed rape in the diet, rather than the amount ingested. It was concluded that the health of roe deer ingesting oilseed rape may be affected if other