Ruminants grazing on metal contaminated past~re may ingest cadmium and lead which accumulates in body tissues. However, only a few studies have investigated their effect on rumen microbes. These studies tend to use the metal in one salt form, which is often different to the form of cadmium or lead found as the environmental contaminant. This thesis aimed to investigate the effects of different cadmium and lead salts by measuring parameters of fermentation and to investigate how these metals are distributed in the rumen fluid to determine their availability to rumen microbes. Rumen fluid was incubated with lead and cadmium as the acetate, chloride, nitrate, oxide and sulphate salts at a range of concentrations. Gas production, volatile fatty acids (VFA), neutral detergent fibre (NDFd), organic matter disappearance (OMd), microbial growth and metal distribution were measured. AIl cadmium salts, except oxide, inhibited gas production although effects varied between salts; . with a negative correlation occurring between gas production and salt solubility at 90 mg Cd/kg DM. Total VFA was only inhibited by cadmium nitrate. Other parameters were not affected. Lead chloride, nitrate and sulphate increased gas production and OMd at all lead concentrations. Lead oxide increased gas production at 400 mg Pb/kg DM. Lead acetate had no effect on gas production. Total and individual VFA's were affected by each salt although this was not related to salt solubility. The insoluble lead salts inhibited microbial growth. The majOlity of both metals were recovered from the non bacterial fractions of the rumen fluid. Low propoltions were recovered from the bacterial ceIl waIls, with a higher concentration of each metal entering the ceIl contents. Salt type did not affect the distribution of either metal. OveraIl, cadmim'n toxicity to rumen microbes was dependent on concentration and salt solubility and lead displayed potential stimulation.