Rumen and caecal digesta were collected, under anaesthetic, from eight sheep offered either hay, pelleted concentrate or pasture at the Johnston Memorial Laboratory, Lincoln University during 1991. Subsamples of digesta were incubated at 39°C for 1 h after adjustment of pH within the range 0·5–12 by the addition of H2SO4 or NaOH. The samples were centrifuged at 30000 g for 30 min and magnesium (Mg) concentration measured in the 30000 g supernatant fraction and in total digesta to assess Mg solubility. In rumen digesta, Mg solubility declined from 0·86 at pH 5 to 0·30 at pH 7 and differences in response between diets were small. Magnesium solubility in caecal digesta was generally higher than in ruminal digesta, and particularly at pH values >6. At pH 7 the difference was twofold. Moreover, differences were observed between diets in the rate of decline in solubility in caecal digesta with increasing pH. At pH 5, 0·90 of Mg from hay and concentrate diets was soluble compared with only 0·8 for pasture. At pH 7, Mg solubility in caecal digesta from hay and concentrate-fed animals was almost double that from pasture-fed animals (0·64 and 0·62 v. 0·36, respectively). The implications of the findings for Mg homoeostasis in ruminants are discussed.