The effect of Garcinia kola seed extract (100 mg/kg) on the pharmacokinetic and antibacterial effects of ciprofloxacin hydrochloride (40 mg/kg) was studied. The results (mean +/- SEM) indicated that concurrent administration of both agents significantly (P < 0.05) decreased average serum concentration, peak serum concentration, and elimination rate of ciprofloxacin HCl, whereas the half-life and clearance rate were increased. The decrease in clearance rate was not significant. There was no difference in time to peak plasma concentration of ciprofloxacin HCl in both groups (n = 5), which occurred at 1 hour. However, the peak plasma concentration of ciprofloxacin HCl was 46.90 +/- 9.50 microg/mL in the group that received ciprofloxacin HCl alone as against 35.80 +/- 9.30 microg/mL noted in the group that received both agents (difference of 22.24%). At 2.5 hours and longer, the values were higher in the group that received both agents, but these were not statistically significant. The reciprocal serum inhibitory titer (SIT) was 33.33 and 50.00% higher in group that received ciprofloxacin HCl alone at 1 and 2.5 hours, respectively; the highest value for both groups being at 1 hour. In contrast, at 4 hours, the value of reciprocal SIT was 66.67% higher in the group that received both agents and at 24 hours, the value was zero for both groups. The observed pharmacokinetic and antibacterial interactions at various time interval indicate biphasic interaction. The interaction was antagonistic at 1 and 2.5 hours, but exhibited potentiation at 4 hours. The precise mechanism underlying the observed biphasic interaction is not fully understood.