EXHIBIT 5.1

[Cooley Godward LLP Letterhead]

May 23, 2000

Inhale Therapeutic Systems
150 Industrial Road
San Carlos, CA

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Inhale Therapeutic Systems, Inc. (the "Company") of a
Registration Statement on Form S-3 on or about May 1, 2000 (the "Registration
Statement") with the Securities and Exchange Commission covering the offering of
up to $230,000,000 aggregate principal amount of 5% Convertible Subordinated
Notes due February 8, 2007 (the "Notes") and 2,998,305 shares of the Company's
Common Stock, $.0001 par value issuable upon converstion of the Notes (the
"Shares" and together with the Notes, the "Securities").

In connection with this opinion, we have examined the Registration Statement and
related Prospectus, your Certificate of Incorporation and Bylaws, as amended,
and such other documents, records, certificates, memoranda and other instruments
as we deem necessary as a basis for this opinion. We have assumed the
genuineness and authenticity of all documents submitted to us as originals, the
conformity to originals of all documents submitted to us as copies thereof, and
the due execution and delivery of all documents where due execution and delivery
are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Securities, when sold and issued in accordance with the Registration
Statement and related Prospectus, will be validly issued, fully paid, and
nonassessable.

We consent to the filing of this opinion as an exhibit to the Registration
Statement.

Very truly yours,

Cooley Godward LLP

/s/ Mark P. Tanoury

Mark P. Tanoury