This paper considers solutions u
α
of the three-dimensional Navier–Stokes equations on the periodic domains Q
α
≔ (−α,α)3 as the domain size α → ∞, and compares them to solutions of the same equations on the whole space. For compactly-supported initial data
u
α
0
∈
H
1
(
Q
α
)
, an appropriate extension of u
α
converges to a solution u of the equations on
R
3
, strongly in
L
r
(
0
,
T
;
H
1
(
R
3
)
)
, r ∈ [1, ∞). The same also holds when
u
α
0
is the velocity corresponding to a fixed, compactly-supported vorticity. A consequence is that if an initial compactly-supported velocity
u
0
∈
H
1
(
R
3
)
or an initial compactly-supported vorticity
ω
0
∈
H
1
(
R
3
)
gives rise to a smooth solution on [0, T*] for the equations posed on
R
3
, a smooth solution will also exist on [0, T*] for the same initial data for the periodic problem posed on Q
α
for α sufficiently large; this illustrates a ‘transfer of regularity’ from the whole space to the periodic case.