In the first part of this paper, we consider the problem of fill-in of nonnegative scalar curvature (NNSC) metrics for a triple of Bartnik data (Σ, γ, H). We prove that given a metric γ on S n−1 (3 ≤ n ≤ 7), (S n−1 , γ, H) admits no fill-in of NNSC metrics provided the prescribed mean curvature H is large enough (Theorem 1.4). Moreover, we prove that if γ is a positive scalar curvature (PSC) metric isotopic to the standard metric on S n−1 , then the much weaker condition that the total mean curvature ´Sn−1 H dµ γ is large enough rules out NNSC fill-ins, giving an partially affirmative answer to a conjecture by Gromov (see P. 23 in [12]). In the second part of this paper, we investigate the θ-invariant of Bartnik data and obtain some sufficient conditions for the existence of PSC fill-ins.