The Portashrinker is a handheld device invented by Ms. Frizzle that shoots a beam that shrinks or causes the target to grow. Ms. Frizzle often keeps the portashrinker in her dress pocket, for a sudden field trip idea or to get out of a "large" situation.
The portashrinker is made of recycled plastic, and was among the items on the bus un-recycled by Wanda's wish. ("Holiday Special") It is sensitive to water, and was unable to work after falling into a puddle until it had dried. The bus has a device that holds the portashrinker while it dries, and "dings when the portashrinker is too wet to work" called the doodinger. ("Butterfly and the Bog Beast")
Once while the class visited Wanda's house, her little brother William got into Ms. Frizzle's tool box and shrunk the class with the portashrinker. He later stepped on it, breaking it. Mrs. Li found it afterwards, and threw it in the toybox. ("Under Construction")
- Whether this is the only portashrinker is unknown. Later episodes show a portashrinker being used, but the series does not necessarily follow chronological order.