Rat Spleens

• Type: Potion Ingredient
• Source: Rat

The novels do not provide a detailed physical description of rat spleens. As a Potion Ingredient, they are presumably harvested from rats and are likely preserved, either dried or kept in liquid, for storage in a Potions classroom or apothecary. They are stored in the student store cupboard within the Potions Classroom at Hogwarts.

Rat spleens are a key ingredient in the brewing of Shrinking Solutions. The quantity used is critical to the success and safety of the potion. Adding too many rat spleens will cause the Shrinking Solution to become poisonous. This property highlights the precision required in Potions and the potential dangers of incorrectly prepared concoctions.

Rat spleens are notably mentioned during a third-year Potions lesson taught by Professor Severus Snape in Harry Potter and the Prisoner of Azkaban.

• During this class, students are tasked with brewing a Shrinking Solution. Neville Longbottom struggles with his potion, nearly melting his cauldron.
• When Professor Snape critiques Neville's work, he quizzes Hermione Granger on what would happen if he were to add too many rat spleens to the mixture. Hermione correctly states that the potion would turn poisonous.
• Professor Snape later penalizes Harry Potter for attempting to advise Neville, ordering Harry to fetch him “one rat spleen” from the store cupboard.

This scene serves to illustrate the strict and often unforgiving nature of Potions as a discipline, as well as reinforcing the character dynamics between Snape, Harry, Neville, and Hermione.

• In the film adaptation of Harry Potter and the Prisoner of Azkaban, the Potions scene involving the Shrinking Solution and the mention of rat spleens is included, closely following the events of the book. (film)
• Rat spleens also appear as a collectible Potion Ingredient in various Harry Potter video games, particularly those where potion-brewing is a gameplay mechanic. (video game)
• The official Wizarding World website (formerly Pottermore) confirms that rat spleens are an ingredient in the Shrinking Solution. (Pottermore)