In this paper we study the global well-posedness of the following Cauchy problem on a sub-Riemannian manifold M :for u 0 ≥ 0, where L M is a sub-Laplacian of M . In the case when M is a connected unimodular Lie group G, which has polynomial volume growth, we obtain a critical Fujita exponent, namely, we prove that all solutions of the Cauchy problem with u 0 ≡ 0, blow up in finite time if and only if 1 < p ≤ p F := 1 + 2/D when f (u) ≃ u p , where D is the global dimension of G. In the case 1 < p < p F and when f : [0, ∞) → [0, ∞) is a locally integrable function such that f (u) ≥ K 2 u p for some K 2 > 0, we also show that the differential inequalitydoes not admit any nontrivial distributional (a function u ∈ L p loc (Q) which satisfies the differential inequality in D ′ (Q)) solution u ≥ 0 in Q := (0, ∞)×G. Furthermore, in the case when G has exponential volume growth and f : [0, ∞) → [0, ∞) is a continuous increasing function such that f (u) ≤ K 1 u p for some K 1 > 0, we prove that the Cauchy problem has a global, classical solution for 1 + 2/d < p < ∞ and some positive u 0 ∈ L q (G) with 1 ≤ q < ∞, where d is the local dimension of G. Moreover, we also discuss all these results in more general settings of sub-Riemannian manifolds M .