Hello Laura,

The weight of the mouse is a key factor in determining the appropriate size (gauge and length) of the gavage needle used for oral administration of substances. Generally, smaller mice require smaller gauge needles, while larger mice can tolerate larger gauge needles.

Postnatal day 21 C57BL/6 mice are likely to fall within the weight range of 15-20 grams for which a 22-gauge needle is typically used. You will also need to know the length of the needle to ensure proper delivery of the substance into the stomach without causing damage or discomfort. For this age group, a needle length of 1 to 1.5 inches is generally suitable. 

Best,

Hello Malcolm! Thank you so much for the answer!