Oral ampicillin and penicillin V was given to 23 and 42 children, respectively, with upper respiratory infections. The absorption of the drugs was followed on one of the first days of treatment for 3 hours after administration by taking capillary samples and determining the serum levels. Approximately 60% of the children were tested for oral absorption during convalescence, 6-8 days after the initiation of the treatment. Diarrhea was evident in more than 60% of the children during treatment with both ampicillin and penicillin V. Diarrhea of short duration did not have influence on the absorption of the antibiotics. Only when persistent diarrhea over the whole therapy period existed, absorption of penicillin V was decreased.