Two years ago, when the U.S. Congress approved a major rewrite of the nation’s chemical safety law, lawmakers ordered federal regulators to take steps to reduce the number of animals that companies use to test compounds for safety. But a recent analysis by two animal welfare groups found that the number of animal tests requested or required by the Environmental Protection Agency (EPA) jumped dramatically last year, from just a few dozen tests involving fewer than ۷۰۰۰ animals in ۲۰۱۶, to more than ۳۰۰ tests involving some ۷۵,۰۰۰ rats, rabbits, and other vertebrates.

The cause of the increase isn’t clear. But the new law imposes stricter requirements on a broader array of chemicals than its predecessor, including both new products and ones already on the market, and experts say EPA staff may be trying to comply by gathering more test data from companies. Both industry and animal welfare groups are alarmed by the trend, and are asking agency officials to clarify why they are requesting the tests—and how they plan to reduce the number in the future.

In a ۲۷ March letter to EPA officials, the two Washington, D.C.–based groups that produced the analysis—People for the Ethical Treatment of Animals (PETA) and the Physicians Committee for Responsible Medicine (PCRM)—wrote that the “appalling” number of animals being used in tests “indicates EPA is failing to balance” its responsibility to evaluate chemicals’ risks against its obligation to pursue alternatives to animal testing.

In ۲۰۱۶, many animal welfare activists applauded lawmakers for including a provision in a major rewrite of the ۱۹۷۶ Toxic Substances Control Act (TSCA) requiring EPA to develop a plan to “reduce, refine or replace” the use of vertebrate animals in testing. Lawmakers suggested the agency could save time and money by harnessing advances in computer modeling, biochemistry, and cell-based testing methods to replace test animals. They ordered EPA to finalize a long-term strategy for increasing the use of such alternatives by this year.

EPA released a draft of that strategy for public comment in March. In preparing a response, PETA and PCRM used a government database to tally the agency’s TSCA-related animal tests over the past ۳ years . In ۲۰۱۵, EPA required or requested ۲۱ tests involving ۸۸۸۱ animals, the groups found; in ۲۰۱۶, it asked for ۳۷ tests involving ۶۵۳۹ animals. In ۲۰۱۷, the first full year that the new law was in force, the numbers jumped to ۳۳۱ tests and ۷۶,۵۲۳ animals. Some tests involve rats inhaling substances, whereas others call for placing chemicals into the eyes of rabbits.

The two groups argue that the agency hasn’t adequately explained why it can’t obtain the needed data from nonanimal tests. A major chemical industry advocacy group, the Washington, D.C.–based American Chemistry Council (ACC), echoes that concern. EPA sometimes appears “unwilling” to rely on data from computational modeling, for example, “even when it is generated from agency-recommended programs,” says Jon Corley, an ACC spokesperson.

EPA did not respond to a request for comment on the surge in testing or what might be driving it. One factor might be that EPA staff are not yet fully aware of proven alternatives to animal tests, says Kristie Sullivan, PCRM’s vice president of research policy. They might need more training and funding “to stay abreast of new developments in toxicology, so that they can quickly incorporate new methods and kinds of data into their decision-making process,” she says.

Other groups, however, are urging patience in allowing EPA to pursue alternatives to animal testing while adapting to the new law. “We need to ensure that the alternative testing methods that are implemented are able to actually identify toxicity, exposure and potential adverse effects of chemicals,” says Daniel Rosenberg, an attorney with the Natural Resources Defense Council in Washington, D.C. “That’s not something that was ever going to happen overnight. We need to bend the curve slowly over time as science evolves.”

EPA’s views on the matter could become clearer soon. The comment period on its draft strategy for reducing animal tests closes Friday, and the agency is required to issue a final plan by ۲۲ June.