As part of a study addressing chronic alcohol consumption and simian immunodeficiency virus, 31 rhesus macaques (Macaca mulatta) were implanted with gastric catheters used to deliver alcohol or isocaloric sucrose (control). Once implanted, the animals wore jackets and were housed in specialized cages modified with swivels and tethers. During the course of the study, 3 animals developed clinical signs indicating possible instability of the implanted gastric catheter. All 3 animals were found to have a string foreign body wrapped around the distal end of the catheter, with 2 of the catheters perforating the intestinal wall. Gastroscopy was used to screen remaining animals to determine catheter position and the presence of a foreign body attached to the end of the catheter. Results of the screening revealed that of the 28 remaining animals, 9 had malpositioned catheters; string foreign bodies were associated with 3 of the 9 malpositioned catheters. We initially hypothesized that the peristaltic motion of the stomach, combined with the attachment of string, which was probably ingested by subjects after manipulating their jackets, led to eventual catheter displacement. We later concluded that the string may have played a secondary role but was not the primary cause of catheter instability, because several malpositioned catheters had no string attached at the time of diagnosis. Subsequent modifications were instituted, including modifying the surgical technique, altering the type of gastric catheter used, and increasing environmental enrichment for animals with known tendency to manipulate their jackets.