The efficiency of equipment suitable for the inhalation of drugs by calves was assessed in six animals which inhaled radioisotopically labelled particles while suffering from reversible diffuse bronchoconstriction induced experimentally with 5-hydroxytryptamine and while they were breathing normally. Respiratory rates and data from pulmonary function tests and scintiscans were recorded during both investigations. After the first investigation, a mean (se) wash-out period of 9.8 (3.2) days was allowed. Under diffuse bronchoconstriction, the respiratory rate, the oscillatory resistance and the compliance of the respiratory system reached 282.1 (22.0), 161.1 (10.8) and 68.8 (2.7) per cent of their respective baseline values. When the calves were breathing normally these parameters did not change over time. The ratios (Cp/Ct) of the counts of γ-disintegrations in the peripheral part (Cp) of the lungs and in the total lung area (Ct) were not significantly different when comparing the results from the two investigations. The ratios of Cp/Ct in the left lungs did not differ significantly from those in the right lungs.