Effects of sodium bicarbonate (NaHCO3-) ingestion on improving performance or recovery are not clear. Several studies have demonstrated both ergogenic and non ergogenic effects of NaHCO3- ingestion on high-intensity exercise performance, or improvements in perceptions of effort and recovery (1). Despite the use of Heart Rate Variability (HRV) as a marker of recovery or even precompetitive anxiety (2), there are not studies focused on the possible effects of NaHCO3- on HRV during BMX races. The aim of the study was to examine the possible effects of NaHCO3- on HRV indexes among a field test simulating a BMX race. Ten elite BMX riders (19.2±3.4 years, 72.4±8.4 kg, 174.2±5.4 cm) participated in this study consisting in two trials performed in separated days. Each trial included three consecutive BMX heats in an Olympic track separated by 15 min recovery. Ninety minutes prior to exercise subjects ingested either NaHCO3- (0.3 g.kg-1 body weight) or placebo. The two treatments conditions were administered in a counterbalanced, crossover, randomly assigned and double-blind manner. Heart Rate Variability was measured each day at rest using a Polar RS800 monitor during 15 min, just before ingesting the substance (PRE) and also after finishing the third heat (POS). Polar Pro Trainer 5 software (Polar Electro, Kempele, Finland) and each downloaded R-R interval file was then further analyzed by means of Kubios HRV Analysis Software 2.0 (The Biomedical Signal and Medical Imaging Analysis Group, University of Kuopio, Finland). Two way ANOVA with repeated measures were used to determine differences between treatments at each exercise moment. The descriptive results in PRE for placebo vs. NaHCO3- conditions respectively were (mean±SD): MeanRR 882±122 vs. 830±123; SDNN 70±27 vs. 57±24; rMSSD 65±31 vs. 50±31; LnLF 7.7±0.7 vs. 7.2±0.6; LnHF 7.2±1 vs. 6.7±1.2; LF/HF 3.2±5.7 vs. 2.7±2.7; SD1 46.6±22.5 vs. 36.3±22.6; SampEn 1.4±0.2 vs. 1.4±0.3; DFA1 1.1±0.2 vs. 1.2±0.3. In POS, the results for placebo vs. NaHCO3- conditions respectively were (mean±SD): MeanRR 524±43 vs. 539±38; SDNN 6.4±2.4 vs. 8.8±2.0; rMSSD 4.1±1.9 vs. 4.0±1.1; LnLF 2.8±1.2 vs. 3.0±1.0; LnHF 0.9±0.8 vs. 1.2±1.0; LF/HF 8.3±4.1 vs. 10.9±9.5; SD1 2.9±1.4 vs. 2.9±0.8; SampEn 0.8±0.2 vs. 0.8±0.3; DFA1 1.4±0.3 vs. 1.5±0.1. Despite in POS no significant differences were found between both conditions (p>0.05), after the NaHCO3- ingestion before the tests started (PRE) significant differences were found for some important HRV indexes as MeanRR (p=0.043), SDNN (p=0.005), rMSSD (p=0.027) and SD1 (p=0.027), all of them in favor of the placebo condition (higher HRV). So, besides any HRV indexes were not improved in NaHCO3- condition, some important indexes got worse may be because of subjects’ feeling and effects related to NaHCO3- ingestion.