Original Message

61.3 (c) Medical certificate.

(1) A person may serve as a required pilot flight  crewmember> of an  aircraft only if that person holds the appropriate  medical certificate issued under part 67 of this chapter, or other documentation acceptable to the  FAA, that is in that  person's physical possession or readily accessible in the  aircraft. Paragraph (c)(2) of this section provides certain exceptions> to the requirement to hold a  medical certificate.

https://www.govinfo.gov/content/pkg/CFR-2011-title14-vol2/pdf/CFR-2011-title14-vol2-sec61-3.pdf

§ 61.23 (b) Medical certificates: Requirement and duration.

Must hold at least a third-class medical certificate -