This paper describes the performance of a prototype telemetric intraruminal bolus that measures and records pH continuously, can be delivered orally to the reticulum via bolling gun, has no external attachments and allows unrestricted activity of the animal. When interrogated by wireless the bolus transmits the recorded data to an operator standing beside the animal with a handheld receiving station. Boluses were placed in fistulated animals to enable direct comparison with samples obtained directly from the rumen and measured with a laboratory instrument. Overall, the mean (±s.d.) pH recorded on the manually collected samples (pH 6.64 ± 0.67) was generally less than that of the continuously measured telemetric system (pH 7.03 ± 0.54) with a correlation of r = 0.93 (P < 0.01). Data are presented to show typical diurnal and grain-enforced changes in pH recorded in a rumen over a 70-day period. The development of the Well Cow pH bolus device potentially enables researchers, dairy farmers and feedlot managers to monitor rumen function of any ruminant over prolonged periods without the need for invasive sampling. Enemark et al. (2003) considered that a 14–21-day observational period is required to properly monitor for conditions such as subacute ruminal acidosis. Whilst significant correlation (P < 0.01; r = 0.982) existed between the two readings for the first 40 days of continuous recording, the Well Cow pH bolus reading started to deviate significantly from the directly measured value thereafter. Regardless, a continuously measuring functional life of up to 40 days indicates that the current prototype has the capacity to accurately detect subacute ruminal acidosis.