Short answer. No. Under Article 1643, a lease of things exists only where one party binds himself to give another the use or enjoyment of a thing for a price certain. The rent is an essential element. Without an agreed price, there is no valid lease of things — at most some other arrangement, but not the lease itself.
What the law says
one of the parties binds himself to give to another the enjoyment or use of a thing for a price certain, and for a period which may be definite or indefinite
Civil Code, Article 1643 — Lease of Things, and the Ninety-Nine-Year Ceiling. Read the full provision →
Rent is part of the definition
Article 1643 defines the lease of things by its very elements. In such a lease, one of the parties binds himself to give to another the enjoyment or use of a thing for a price certain, and for a period which may be definite or indefinite. Three things are built into that sentence: a thing to be used, a price to be paid, and a period of use. The price certain — the rent — is not an optional extra. It is one of the pillars the contract stands on. Remove it, and what remains does not fit the law's idea of a lease of things at all.
What happens if there is no agreed price
If the parties genuinely never fixed a rent, there is no meeting of minds on an essential element, and no lease of things comes into being. The arrangement may be something else entirely. Letting someone use a thing for free, expecting only its return, looks more like a gratuitous loan for use than a lease, because the defining feature of a lease — payment for use — is missing. Calling a document a "lease" does not make it one; the law looks at whether the essential elements are present. So a supposed lease with no price to pay is, in law, not a lease that can be enforced as such.
'Certain' does not mean rigidly stated
Price certain does not require that a peso figure be spelled out word for word. A rent is certain enough if the parties agreed on a definite amount, or on a clear basis for computing it that needs no further bargaining — for instance, a rate tied to an agreed measure. What defeats the lease is the absence of any agreed price at all, or terms so vague that the amount cannot be worked out without the parties negotiating afresh. The period, by contrast, is more flexible: the same article accepts a term that is definite or indefinite, so uncertainty about duration alone will not sink an otherwise complete lease.
The outer limit on duration
Article 1643 also caps how long a lease of things may run. It provides that no lease for more than ninety-nine years shall be valid. This ceiling prevents a lease from tying up a thing across generations to the point of resembling a permanent disposition rather than a temporary use. So while the period may be indefinite, it cannot exceed that outer limit. Together with the requirement of a price certain, the article marks the boundaries of a valid lease of things: there must be a real rent, and the term cannot stretch beyond ninety-nine years. Miss the first, and there is simply no lease to enforce.