diff --git a/google/cloud/texttospeech/v1/cloud_tts_lrs.proto b/google/cloud/texttospeech/v1/cloud_tts_lrs.proto
index 6f9662355..a903eb63f 100644
--- a/google/cloud/texttospeech/v1/cloud_tts_lrs.proto
+++ b/google/cloud/texttospeech/v1/cloud_tts_lrs.proto
@@ -56,22 +56,23 @@ service TextToSpeechLongAudioSynthesize {
 // `SynthesizeLongAudio` method.
 message SynthesizeLongAudioRequest {
   // The resource states of the request in the form of
-  // `projects/*/locations/*/voices/*`.
+  // `projects/*/locations/*`.
   string parent = 1;
 
   // Required. The Synthesizer requires either plain text or SSML as input.
+  // While Long Audio is in preview, SSML is temporarily unsupported.
   SynthesisInput input = 2 [(google.api.field_behavior) = REQUIRED];
 
   // Required. The configuration of the synthesized audio.
   AudioConfig audio_config = 3 [(google.api.field_behavior) = REQUIRED];
 
-  // Specifies a Cloud Storage URI for the synthesis results. Must be
+  // Required. Specifies a Cloud Storage URI for the synthesis results. Must be
   // specified in the format: `gs://bucket_name/object_name`, and the bucket
   // must already exist.
-  string output_gcs_uri = 4;
+  string output_gcs_uri = 4 [(google.api.field_behavior) = REQUIRED];
 
-  // The desired voice of the synthesized audio.
-  VoiceSelectionParams voice = 5;
+  // Required. The desired voice of the synthesized audio.
+  VoiceSelectionParams voice = 5 [(google.api.field_behavior) = REQUIRED];
 }
 
 // The message returned to the client by the `SynthesizeLongAudio` method.
diff --git a/google/cloud/texttospeech/v1beta1/cloud_tts_lrs.proto b/google/cloud/texttospeech/v1beta1/cloud_tts_lrs.proto
index ecfecf0a4..c68a1a2ac 100644
--- a/google/cloud/texttospeech/v1beta1/cloud_tts_lrs.proto
+++ b/google/cloud/texttospeech/v1beta1/cloud_tts_lrs.proto
@@ -56,22 +56,23 @@ service TextToSpeechLongAudioSynthesize {
 // `SynthesizeLongAudio` method.
 message SynthesizeLongAudioRequest {
   // The resource states of the request in the form of
-  // `projects/*/locations/*/voices/*`.
+  // `projects/*/locations/*`.
   string parent = 1;
 
   // Required. The Synthesizer requires either plain text or SSML as input.
+  // While Long Audio is in preview, SSML is temporarily unsupported.
   SynthesisInput input = 2 [(google.api.field_behavior) = REQUIRED];
 
   // Required. The configuration of the synthesized audio.
   AudioConfig audio_config = 3 [(google.api.field_behavior) = REQUIRED];
 
-  // Specifies a Cloud Storage URI for the synthesis results. Must be
+  // Required. Specifies a Cloud Storage URI for the synthesis results. Must be
   // specified in the format: `gs://bucket_name/object_name`, and the bucket
   // must already exist.
-  string output_gcs_uri = 4;
+  string output_gcs_uri = 4 [(google.api.field_behavior) = REQUIRED];
 
-  // The desired voice of the synthesized audio.
-  VoiceSelectionParams voice = 5;
+  // Required. The desired voice of the synthesized audio.
+  VoiceSelectionParams voice = 5 [(google.api.field_behavior) = REQUIRED];
 }
 
 // The message returned to the client by the `SynthesizeLongAudio` method.