Recent results in normotensive Wistar-Kyoto (WKY) rats show that nonlinear method may be more specific to quantify sympathetic and parasympathetic activities than the low (LF) and high frequencies (HF) spectral powers of blood pressure (BP) and R-R interval (RR). The present study extends this conclusion to spontaneously hypertensive rats (SHR). Blood pressure was recorded for 30 min before and after intravenous injection of saline, hexamethonium, atropine, atenolol, or prazosin. Mean level, standard deviation (SD), spectral LF and HF components, and three nonlinear indexes (percentage of recurrence, percentage of determinism, and length index of the recurrence plot method) were used to analyze the BP and RR signals. In conscious SHR, sympathetic but not parasympathetic blockade reduced BP level and LF-BP, and increased nonlinear indexes of BP. RR increased after -sympathetic and ganglionic blockade, decreased after parasympathetic blockade, and remained unchanged after ₁-sympathetic blockade. SD-RR decreased after ganglionic and ₁ blockade, whereas HF-RR increased after -sympathetic blockade. The effects on nonlinear indexes of RR are clear and consistent: only ₁-blockade increased the indexes. Our nonlinear indexes may be useful to investigate cardiovascular functions in normotension and hypertension.