To obtain more information related to the inhibitive effect of anesthetic on spontaneous firing of neurons, several kinds of experimental data were collected before and after applying 10 mmol/L urethane, which included spontaneous firing, sodium current (I_Na), delayed rectifier potassium current (I_K) and transient outward potassium current (I_A) of hippocampal CA1 pyramidal neurons in rats. All of the experiments were performing on hippocampal slices in vitro. Sample entropy (SampEn) and detrended fluctuation analysis (DFA) were employed to measure the complexity and fractal property of spontaneous firing of neurons. It was found that the spontaneous firing rate was significantly decreased, the entropy of ISI sequence was obviously reduced, and the long-range temporal correlations of ISI sequence were weakened after performing 10 mmol/L urethane. Furthermore, I_Na was significantly inhibited, but I_K and I_A were not statistically changed for 10 mmol/L urethane. This suggests that the fractal properties and complexity of underlying dynamics of the system have been reduced by urethane. And the reduction of complexity may because of inhibition of I_Na. The results demonstrate that apparently random fluctuations in neuron spontaneous firing are dictated by a complex deterministic process that imparts “long-term” memory to the dynamic system.