Students' mathematical learning is bolstered when they have caring relationships with their teachers, yet there is little research on how learning environments may constrain or facilitate teacher care. In this study, I investigate how secondary mathematics teachers understood their care for students online during the COVID-19 pandemic. Through analysis of interview and focus group transcripts, I theorize the role of the learning environment in how teachers care for students and their dispositions toward that work. Findings show how an online learning environment both constrained and facilitated teacher care. An obscuration of student thinking online made it difficult for teachers to care for students' mathematical thinking. The online learning environment contributed to a curtailment of collaborative problem-solving, which in person had supported teacher care for student-student relationships. Teachers described greater privacy with some students online but difficulty getting to know most students. Teachers also reported their care to be conditional on students' relational initiation or reciprocity. This study highlights the need for professional development for teachers when learning environments shift and future research about learning environments and relational power.