In rodents, postnatal neurogenesis is restricted to certain brain areas including the olfactory bulb. It has been shown that blocking this neurogenesis disrupts olfactory learning, but the techniques used lacked specificity. Here, designer receptors exclusively activated by designer drugs (DREADDs) were used to inactivate newborn neurons at postnatal day 0 (P0; juvenile) and P42 (adult). Inactivation at P42 attenuated the response to a novel appetitive odour, whereas inactivation at P0 blocked a freezing response to an aversive odour, suggesting that neurons born in the early and later postnatal periods play different parts in sensory learning.