Monensin, an ionophore, and Aspergillus oryzae fermentation extract (AO), a fungal feed additive, are growth promoters which modify rumen fermentation. The effects of combining these additives were determined in the rumen simulation technique (Rusitec). Sixteen vessels received 20 g/day of a diet of hay, barley, molasses, fishmeal and a mineral/vitamin mixture (500, 299·5, 100, 91 and 9·5 g/kg DM respectively). AO, monensin (M) and AO + monensin (AO/M) were each added to four vessels at 500, 10 and (500 + 10) mg/day respectively. Both M and AO/M increased propionate (14·5 and 138 v. 91 mmol/day in controls; P < 0·001) and reduced butyrate production (6·6, 5·2 and 9·1 mmol/day respectively; P < 0·01), whereas AO had no effect. AO increased bacterial numbers by 70% (P < 0·055), while M had no effect, and in the presence of M, AO also had no effect. Proteolytic activity in samples from the vessels decreased (P < 0·05) in all treatments (1·06, 0·71, 0·60 and 0·47 mg 14C-casein/mg protein/h for control, AO, M and AO/M respectively). In contrast, deamination of amino acids increased (P < 0·001) with AO and AO/M but decreased slightly with M alone (482, 646, 434 and 644 nmol NH3/mg protein/h). Ammonia output was unchanged (47·4, 57·2, 42·7 and 44·8 mg/day). Thus each additive was dominant over the other for different activities, with monensin generally suppressing the effects of AO rather than vice-versa.