Highly selective vagotomy was performed on five dogs. Postoperatively, gastrin cell (G cell) hyperplasia occurred in all dogs. Mean preoperative G cell numbers increased from 350 to 530/cm mucosal length (p < 0.02). Antral tissue gastrin also increased by 100 per cent (6.7 × 106 to 13.7 × 106 pg/gm tissue, p < 0.05). Basal and stimulated serum gastrin were unchanged following highly selective vagotomy. The cause for G cell hyperplasia is not clear, but is probably multifactorial.