Objective: To determine possible electrolyte changes induced by competitive tennis in young female players. Material and methods: Ten female tennis players (mean ± SEM age: 15.6 ± 0.87 years, height: 164.55 ± 1.98 cm, weight: 58.77 ± 2.08 kg) were selected to participate in an official tennis tournament. A blood sample (15 mL) was obtained 30 minutes before the tournament. After extraction, the players drank 500 mL of water before the match. Water intake (ad libitum) and heart rate (HR) were recorded during the match. The matches were recorded on video. Another blood sample was obtained at the end of the match. The variables analyzed were HR, total play time, real play time, rest time, plasma electrolyte concentrations (sodium, potassium and chloride), and total plasma proteins to determine plasma volume changes. Results: Total play time was 99.6 ± 18.5 min and mean HR during the match was 158.4 ± 1.98 bpm-1. After the match, an increase in plasma volume (3.59 ± 2.56%) and a decrease in body weight (­1.31 ± 0.27) were observed, as well as a significant rise in plasma sodium (136.98 ± 1.71 to 145.77 ± 1.77) and chloride concentrations (92.17 ± 1.21 a 100.12 ± 1.31) compared with those registered before the match (p ≤ 0,001). Plasma potassium concentration decreased slightly (3.90 ± 0.13 to 3.13 ± 0.19) (p ≤ 0.01). Discussion: Hydration should be controlled in tennis players to avoid electrolyte imbalance.