Seasonal rhythmic changes in gentamicin pharmacokinetics was evaluated in 10 healthy male volunteers after single intravenous 80 mg administration of gentamicin at 9:00 a.m. during summer and winter. The mean terminal half-life and AUC of gentamicin were $3.56pm0.14;hr;and;25.03pm2.84;{mu}g/ml{cdot}hr$ in winter and $3.08pm0.41;hr;and;21.84pm2.51;{mu}g/ml{cdot}hr$ in summer. The mean total body clearance $(CL_t)$ and elimination rate constant $(k_{10})$ of gentamicin was $3.17pm0.43;L/hr,;0.458pm0.06;hr^{-1};in;winter;and;3.66pm0.45;L/hr,;0.561pm0.07;hr^{-1}$ in summer, The mean volumn of distribution $(V_{dss})$ of gentamicin at steady state was $12.65pm1.09$L in winter and $12.39pm1.25$ L in summer. Serum concentrations of gentamicin in winter were increased significantly during 4-8 hr (p<0.05) compared to those of gentamicin in summer. The elimination rate constant $(k_{10})$ of gentamicin in winter was decreased significantly $(p<0.05)$ compared to that of gentamicin in summer. The mean volume of distribution at steady state $(V_{dss})$, AUC, mean total body clearance ($CL_t$) and terminal half-life of gentamicin in the winter were increased but were not significant. The mean intrasubject fluctuations in terminal half-life, AUC and $CL_t$ between winter and summer were 8.2, 11.0 and $6.0\%$ respectively.