One hundred and ninety-seven patients with intraabdominal infections were enrolled in a prospective randomized multicenter study of netilmicin administered once daily (n = 98) versus thrice daily (n = 99) in combination with tinidazole administered once daily. Randomization was achieved for the infection site, clinical severity score, daily and total netilmicin dose, and duration of treatment. The mean maximum peak and trough levels of netilmicin in serum were 21.1 and 1.3 mg/l respectively for once daily treated patients, and 10.0 and 2.3 mg/l for thrice daily treated patients (p less than 0.05 for both parameters). The clinical response did not differ between patients treated once daily and those treated thrice daily. Overall rates for clinical cure, improvement and failure of therapy were 77%, 17% and 6% respectively. No significant differences were found between once daily and thrice daily regimens in the occurrence of auditory, vestibular and renal toxicity, overall rates being 5%, 1% and 10% respectively. Impairment of renal function was significantly related to higher maximum netilmicin serum trough levels during therapy, a higher clinical severity score and advanced age. It is concluded that netilmicin given once daily is as effective and safe as the multiple dose regimen. However, monitoring of aminoglycoside serum through levels is still advisable, especially in the old and severely ill patient.