Single units of slowly adapting pulmonary stretch receptors (PSRs) were investigated in anaesthetized cats during spontaneous breathing on continuous positive airway pressure (2-5 cm H₂O), before and after lung lavage and then after instillation of surfactant in order to determine the PSR response to surfactant replacement. PSRs were classified as high-threshold (HT) and lowthreshold (LT), and their instantaneous impulse frequency (f_imp) was related to transpulmonary pressure (P_tp) and tidal volume (V_T). Both the total number of impulses and maximal f_imp of HT and LT PSRs decreased after lung lavage (55% and 45%, respectively) in the presence of increased P_tp and decreased V_T. While P_tp decreased markedly and V_T remained unchanged after surfactant instillation, all except one PSR responded with increased total number of impulses and maximal f_imp (42% and 26%, respectively). Some HT PSRs ceased to discharge after lung lavage but recovered after surfactant instillation. The end-expiratory activity of LT PSRs increased or was regained after surfactant instillation. After instillation of surfactant, respiratory rate increased further with a shorter inspiratory time, resulting in a lower inspiratory to expiratory time ratio. Arterial pH decreased (7.31 ± 0.04 vs. 7.22 ± 0.06) and pCO₂ increased (5.5 ± 0.7 vs. 7.2 ± 1.3 kPa) after lung lavage, but were the same after as before instillation of surfactant (pH = 7.21 ± 0.08 and pCO₂ = 7.6 ± 1.4) during spontaneous breathing. In conclusion, surfactant instillation increased lung compliance, which in turn increased the activity of both HT and LT PSRs. A further increase in respiratory rate due to a shorter inspiratory time after surfactant instillation, suggests that the partially restored PSR activity after surfactant instillation affected the breathing pattern.