['In large animals, rate-dependent AV nodal (AVN) properties of conduction are modulated by autonomic inputs. In this study, we investigated whether the properties of AVN conduction and facilitation are altered by autonomic blockade in the mouse and whether this effect is age dependent.', 'Young (age 4-6 weeks; n = 11) and adult (age 8-9 months; n = 11) female mice underwent in vivo electrophysiologic testing, before and after autonomic blockade. After autonomic blockade, the adult mice had significantly longer AVN effective refractory period (AVNERP; 67 +/- 14 msec vs 56 +/- 4 msec, P = 0.05) and functional refractory period (AVNFRP; 81 +/- 10 msec vs 72 +/- 4 msec, P = 0.05). With autonomic blockade, the increase from baseline of AVN Wenckebach cycle length (deltaAVW; 1.8 +/- 8.1 msec vs 8.8 +/- 3.3 msec, P = 0.04), as well as of AVNERP (deltaAVNERP; 3.5 +/- 3.5 msec vs 21.4 +/- 12.6 msec, P = 0.002) and AVNFRP (deltaAVNFRP; 2.3 +/- 3.2 msec vs 12.8 +/- 9.0 msec, P = 0.008), was significantly larger in adult than in young mice. Compared with young mice, adult mice were less likely to exhibit AVN facilitation (44% vs 90%, P = 0.03) and had smaller maximal shortening of AVN conduction times after the "test beat" for any coupling of the "facilitating beat" (4 +/- 4 msec vs 7 +/- 3 msec, P = 0.05).'] Does autonomic blockade unmask maturational differences in rate-dependent atrioventricular nodal conduction and facilitation in the mouse?