The effect of substituting lucerne hay (LH) with ammoniated wheat straw (AWS) in diets for lactating Holstein Friesland cows was investigated in two trials. In Trial 1 four roughage diets containing LH and AWS in the following ratio`s of 100:0; 67:33; 33:67 and 0:100, were offered ad libitum to eight Holstein Friesland cows according to a 4 x 4 Latin square change-over design. A concentrate mixture supplying 162 g crude protein (CP) and 11.5 MJ ME/kg dry matter (DM) was provided to all cows at 1% of starting body mass per day. In Trial 2 two complete diets containing LH or AWS as roughage sources were mixed to contain similar concentrations of energy, CP and neutral detergent fibre (NDF). The AWS diet was then combined with the LH diet in the following ratio`s: 0:100, 25:75, 50:50, 75:25 and 100:0. Diets were fed ad libitum to 10 Holstein Friesland cows according to a 5 x 5 Latin square change-over design. In Trial 1 dry matter intake (DMI) declined linearly (p < 0.01) at a rate of 2.04 kg/d at increasing levels of AWS in the diet. Milk yield (MY) and 4% fat corrected milk yield (FCM) were similarly affected, i.e. 1.0 and 0.97 kg/d (p < 0.01) respectively. The fat and lactose contents of the milk of cows were unaffected although protein content declined linearly (p < 0.01). The reduction in DMI and MY is probably caused by the bulkiness of the diets as reflected by increasing NDF levels of the diets and/or palatability of AWS in comparison to LH. Increasing AWS levels in Trial 2, where NDF content of the experimental diets varied less, also resulted in a linear decrease (p < 0.05) in DMI of 0.43 kg/d. Daily MY was similarly affected, decreasing (p < 0.01) by 0.80 kg/d. The response of milk fat content (MF%) to increased levels of AWS was curvilinear (p < 0.01). The highest predicted MF% of 3.63 was obtained for the 50:50 AWS and LH diet combination. As a result, FCM was unaffected by inclusion level of AWS up to 50% of the diet. At higher AWS levels (75 and 100%) FCM was reduced (p < 0.01). It was more economical to use AWS at R215/t than LH at prices exceeding R375/t. At these prices LH may be completely substituted with AWS in post-peak lactation diets of dairy cows.