diff --git a/tox.ini b/tox.ini
index 19a5c68b0..587f1b26c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,6 +3,7 @@ minversion = 2.0
 envlist = py35,py27,pep8,bandit
 
 [testenv]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
 passenv = OS_*
@@ -15,6 +16,9 @@ deps =
   -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 commands = flake8
 deps = hacking<0.11,>=0.10.0
@@ -32,6 +36,7 @@ deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:py27-func-rabbit]
+basepython = python2.7
 setenv =
     {[testenv]setenv}
     TRANSPORT_DRIVER=rabbit
@@ -41,10 +46,10 @@ commands = pifpaf run rabbitmq --  python setup.py testr --slowest --testr-args=
 setenv =
     {[testenv]setenv}
     TRANSPORT_DRIVER=rabbit
-basepython = python3.5
 commands = pifpaf run rabbitmq --  python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py27-func-kafka]
+basepython = python2.7
 setenv =
     {[testenv]setenv}
     TRANSPORT_URL=kafka://127.0.0.1:9092/
@@ -52,6 +57,7 @@ setenv =
 commands = {toxinidir}/setup-test-env-kafka.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py27-func-amqp1]
+basepython = python2.7
 setenv =
     {[testenv]setenv}
     TRANSPORT_URL=amqp://stackqpid:secretqpid@127.0.0.1:65123//
@@ -61,7 +67,6 @@ setenv =
 commands = {toxinidir}/tools/setup-test-env-amqp1.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py35-func-amqp1]
-basepython = python3.5
 setenv =
     {[testenv]setenv}
     TRANSPORT_URL=amqp://stackqpid:secretqpid@127.0.0.1:65123//
@@ -71,18 +76,22 @@ setenv =
 commands = {toxinidir}/tools/setup-test-env-amqp1.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py27-func-zmq]
+basepython = python2.7
 commands = {toxinidir}/setup-test-env-zmq.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py35-func-zmq]
 commands = {toxinidir}/setup-test-env-zmq.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py27-func-zmq-dyn]
+basepython = python2.7
 commands = {toxinidir}/setup-test-env-zmq-direct-dynamic.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py27-func-zmq-proxy]
+basepython = python2.7
 commands = {toxinidir}/setup-test-env-zmq-proxy.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:py27-func-zmq-pubsub]
+basepython = python2.7
 commands = {toxinidir}/setup-test-env-zmq-pub-sub.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'
 
 [testenv:bandit]
@@ -108,9 +117,14 @@ local-check-factory = oslo_messaging.hacking.checks.factory
 # do not install test-requirements as that will pollute the virtualenv for
 # determining missing packages
 # this also means that pip-missing-reqs must be installed separately, outside
-# of the requirements.txt files
+# of the requirements.txt files.
+# Ignore optional driver components for now (see https://review.openstack.org/#/c/525319/)
 deps = pip_check_reqs
-commands = pip-missing-reqs -d --ignore-module=oslo_messaging* --ignore-file=oslo_messaging/tests/* --ignore-file=tests/ oslo_messaging
+commands = pip-missing-reqs -d --ignore-module=oslo_messaging* \
+                            --ignore-file=oslo_messaging/tests/* \
+                            --ignore-file=oslo_messaging/_drivers/zmq_driver/* \
+                            --ignore-file=oslo_messaging/_drivers/impl_kafka.py \
+                            --ignore-file=tests/ oslo_messaging
 
 [testenv:releasenotes]
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
@@ -121,7 +135,6 @@ deps = bindep
 commands = bindep {posargs}
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt