β-Blockers are used in pregnancy-associated hypertension and in postnatal cardiac arrhythmias, and the neonate may get them in breast milk. We therefore studied the effects of β-adrenergic medication on interrelations between heart rate (HR), respiration, and arterial blood pressure (aBP) in newborn lambs. The influence of sleep state on these cardiorespiratory interrelations was also examined. HR, aBP, and respiration (based on transthorack electrical impedance) were recorded and the sleep state was visually documented in five healthy chronically instrumented newborn lambs before the age of 30 d. Propranolol was given (1 mg/kg). Two-min stationary segments of the three signals were analyzed using a multivariate autoregressive model, which yields oscillations of the signals and intersignal relationships as source contributions. The variabilities of aBP and HR were greatest at the low frequencies (Pediatr Res 29: 272–277, 1991)