Pain control is one of the most important aspects of therapy in aging, critically ill, and hospitalized veterinary patients. Uncontrolled pain in dogs can increase stress, delay recovery, prolong hospitalization, and contribute to complications. Uncontrolled or chronic pain can also be detrimental to a patient’s immune system, which further complicates recovery. Because of nausea, vomiting, or reluctance to swallow, geriatric ill dogs may be unable to take pain medication orally. This usually requires veterinarians to administer pain medication by injection, which can be more expensive, create anxiety in the patient, and is difficult in the home environment. Geriatric dogs often do not tolerate other analgesics such as nonsteroidal anti-inflammatory drugs due to vomiting, diarrhea, or gastrointestinal ulceration, and may exacerbate underlying kidney disease. Opioid pain relievers may not be tolerated well in geriatric dogs due to sedation, dysphoria, anorexia, nausea, and vomiting. Acetaminophen (the active ingredient in Tylenol) is effective in treating mild to moderate pain in dogs, and is often administered orally, but can also be administered by rectal suppository. There are no current data on the absorption of suppository acetaminophen in healthy dogs or ill hospitalized dogs to guide dosages. If absorbed well by this route, this medication would provide a practical and inexpensive approach to pain control in dogs that cannot take medication by mouth. This study will assess the absorption and plasma concentrations of acetaminophen as administered rectally by suppository in both healthy and clinically ill canine patients to determine dosing recommendations as a treatment for pain.