Both nonlinear and fractal properties of beat-to-beat R-R interval variability signal (RRV) of freely moving lizards (Gallotia galloti) were studied in baseline and under autonomic nervous system (ANS) blockade. Nonlinear techniques allowed us to study the complexity, chaotic behavior, nonlinearity, stationarity and regularity over time of RRV. Scaling behavior of RRV was studied by means of fractal techniques. The ANS blockers used were atropine, propranolol, prazosin and yohimbine. The nature of RRV was linear in baseline and under -, ₁- and ₂-adrenoceptor blockades. Atropine changed the linear nature of RRV to nonlinear and increased its stationarity, regularity and fractality. Propranolol increased the complexity and chaotic behavior, and decreased the stationarity, regularity and fractality of RRV. Both prazosin and yohimbine did not change any of the nonlinear and fractal properties of RRV. It is suggested that 1) the use of both nonlinear and fractal analysis is an appropriate approach for studying cardiac period variability in reptiles; 2) the cholinergic activity, which seems to make the ₁-, ₂- and -adrenergic activity interaction unnecessary, determines the linear behavior in basal RRV; 3) fractality, as well as both RRV regularity and stationarity over time, may result from the balance between cholinergic and -adrenergic activities opposing actions; 4) -adrenergic activity may buffer both the complexity and chaotic behavior of RRV, and 5) neither the ₁- nor the ₂-adrenergic activity seem to be involved in the mediation of either nonlinear or fractal components of RRV.