In order to define a reliable method for estimating maximal ventilation in cattle, 12 healthy calves underwent a rebreathing trial and injections of increasing doses of lobeline, a respiratory analeptic. The effects of these tests on the main ventilatory parameters (tidal volume, VT; respiratory frequency, fRand minute volume,.VE) recorded during the 15 s of maximal response were studied and compared. The sharp rise in.VE(4.8 times higher than the resting value) observed during the rebreathing trial was mainly due to an increase in VT. This rise in ventilation was the highest ever reported in calves. Lobeline dose-dependently enhanced ventilation up to a threshold dose of 0.25 mg/kg, which always produced a maximal response. This maximal response (3.7 times higher than the resting value), reflecting both an increase in fR and VT, was reproducible at an interval of 12 h and was highly correlated with that observed during the rebreathing trial (R = 0.98, P< 0.001). These results suggest that: (1) rebreathing trial is a reliable method to induce and measure maximal ventilation in calves; and (2) lobeline administration (0.25 mg/kg) is a reliable means of accurately estimating this variable. Lobeline administration, unlike the rebreathing trial, is safe and easy to standardize, and the test therefore seems to be the preferred way of studying maximal ventilation in calves.