A series of 3-nitropyrazole compounds represent a new class of synthetic antibacterial agents. One member of this series, 1-(2-hydroxyethyl)-3-nitro-4-pyrazolecarboxamide, exhibited an antibacterial spectrum similar to that of nitrofurantoin. The inhibitory concentrations of this nitropyrazole were lower than those required for nitrofurantoin. Single oral doses of 20 mg/kg resulted in peak nitropyrazole concentrations of 5.8 and >1,000 mug/ml in mouse blood and urine, respectively. In dogs, 87% of a 10 mg/kg oral dose was recovered in urine during a 24-h period with a peak serum concentration of 13.6 mug/ml. This nitropyrazole was highly effective against experimental bacterial infections in mice. The low acute toxicity in mice, rats, or dogs and significant antibacterial activity lead to the conclusion that further evaluation of this compound is warranted.