diff --git a/dotenv/tests/integration/api/from_filename_override.rs b/dotenv/tests/integration/api/from_filename_override.rs
index 8b137891..16785590 100644
--- a/dotenv/tests/integration/api/from_filename_override.rs
+++ b/dotenv/tests/integration/api/from_filename_override.rs
@@ -1 +1,94 @@
+use crate::util::*;
+use dotenvy::from_filename_override;
+use dotenvy_test_util::*;
 
+#[test]
+fn no_file_ok() {
+    let testenv = TestEnv::init();
+    test_in_env(&testenv, || {
+        from_filename_override("nonexistent").ok();
+    });
+}
+
+#[test]
+fn no_file_err() {
+    let testenv = TestEnv::init();
+    test_in_env(&testenv, || {
+        let err = from_filename_override("nonexistent.env").unwrap_err();
+        assert_err_not_found(err);
+    });
+}
+
+#[test]
+fn empty_default_file() {
+    let testenv = TestEnv::init_with_envfile("");
+    test_in_env(&testenv, || {
+        assert!(from_filename_override(".env").is_ok());
+    });
+}
+
+#[test]
+fn empty_custom_file() {
+    let mut testenv = TestEnv::init();
+    testenv.add_envfile(".custom.env", "");
+    test_in_env(&testenv, || {
+        assert!(from_filename_override(".custom.env").is_ok());
+    });
+}
+
+#[test]
+fn one_var_custom_file() {
+    let mut testenv = TestEnv::init();
+    testenv.add_envfile(".custom.env", KEYVAL_1);
+    test_in_env(&testenv, || {
+        assert!(from_filename_override(".custom.env").is_ok());
+        assert_env_var(KEY_1, VAL_1);
+    });
+}
+
+#[test]
+fn override_existing_var_custom_file() {
+    let mut testenv = TestEnv::init();
+    testenv.add_env_var("FOOO", "from_env");
+    testenv.add_envfile(".custom.env", "FOOO=from_file");
+    test_in_env(&testenv, || {
+        assert_env_var("FOOO", "from_env");
+        from_filename_override(".custom.env").unwrap();
+        assert_env_var("FOOO", "from_file");
+    });
+}
+
+#[test]
+fn default_override() {
+    test_in_default_env(|| {
+        from_filename_override(".env").unwrap();
+        assert_env_var(DEFAULT_TEST_KEY, DEFAULT_TEST_VALUE);
+        assert_env_var(DEFAULT_EXISTING_KEY, DEFAULT_OVERRIDING_VALUE);
+    })
+}
+
+#[test]
+fn substitute_self() {
+    let mut testenv = TestEnv::init();
+    testenv.add_env_var("FOOO", "test");
+    testenv.add_envfile(".custom.env", "FOOO=$FOOO+1");
+    test_in_env(&testenv, || {
+        assert_env_var("FOOO", "test");
+        from_filename_override(".custom.env").unwrap();
+        assert_env_var("FOOO", "test+1");
+    });
+}
+
+#[test]
+fn substitute_self_two_files() {
+    let mut testenv = TestEnv::init();
+    testenv.add_env_var("FOOO", "test");
+    testenv.add_envfile(".custom1.env", "FOOO=$FOOO+1");
+    testenv.add_envfile(".custom2.env", "FOOO=$FOOO+1");
+    test_in_env(&testenv, || {
+        assert_env_var("FOOO", "test");
+        from_filename_override(".custom1.env").unwrap();
+        from_filename_override(".custom2.env").unwrap();
+        assert_env_var("FOOO", "test+1+1");
+    });
+}