We consider Kolmogorov-type equations on a rectangle domain (x, v) ∈ Ω = T × (−1, 1), that combine diusion in variable v and transport in variable x at speed v γ , γ ∈ N * , with Dirichlet boundary conditions in v. We study the null controllability of this equation with a distributed control as source term, localized on a subset ω of Ω.In dimension one, when the control acts on a horizontal strip ω = T × (a, b) with 0 < a < b < 1, then the system is null controllable in any time T > 0 when γ = 1, and only in large time T > Tmin > 0 when γ = 2 (see [10]). In this article, we prove that, when γ > 3, the system is not null controllable (whatever T is) in this conguration. This is due to the diusion weakening produced by the rst order term.When the control acts on a vertical strip ω = ω1 × (−1, 1) with ω1 ⊂ T, we investigate the null controllability on a toy model, where (∂x, x ∈ T) is replaced by1/2 , x ∈ Ω1), and Ω1 is an open subset of R N . As the original system, this toy model satises the controllability properties listed above. We prove that, for γ = 1, 2 and for appropriate domains (Ω1, ω1), then null controllability does not hold (whatever T > 0 is), when the control acts on a vertical strip ω = ω1 × (−1, 1) with ω1 ⊂ Ω1. Thus, a geometric control condition is required for the null controllability of this toy model. This indicates that a geometric control condition may be necessary for the original model too.